Zeolite Programming Language

Zeolite is a statically-typed, general-purpose programming language. The type system revolves around defining objects and their usage patterns.

Zeolite prioritizes making it easy to write maintainable and understandable code. This is done by rethinking standard language idioms and limiting flexibility in some places while increasing it in others. In particular, emphasis is placed on the user's experience when troubleshooting code that is incorrect.

The design of the type system and the language itself is influenced by positive and negative experiences with Java, C++, Haskell, Python, Ruby, and Go, with collaborative development, and with various systems of code-quality enforcement.

Due to the way GitHub renders embedded HTML, the colors might not show up in the syntax-highlighted code in this document. If you use the Chrome browser, you can view the intended formatting using the Markdown Viewer extension to view the raw version of this document.

Project Status

Zeolite is still evolving in all areas (syntax, build system, etc.), and it still lacks a lot of standard library functionality. That said, it was designed with practical applications in mind. It does not prioritize having impressive toy examples (e.g., merge-sort or "Hello World" in one line); the real value is seen in programs with higher relational complexity.

Language Overview

This section discusses some of the features that make Zeolite unique. It does not go into detail about all of the language's features; see Writing Programs and the full examples for more specific language information.

Programming Paradigms

Zeolite currently uses both procedural and object-oriented programming paradigms. It shares many features with Java, but it also has additional features and restrictions meant to simplify code maintenance.

Parameter Variance

The initial motivation for Zeolite was a type system that allows implicit conversions between different parameterizations of parameterized types. A parameterized type is a type with type "place-holders", e.g., templates in C++ and generics in Java.

Java and C++ do not allow you to safely convert between different parameterizations. For example, you cannot safely convert a List<String> into a List<Object> in Java. This is primarily because List uses its type parameter for both input and output.

Zeolite, on the other hand, uses declaration-site variance for each parameter. (C# also does this to a lesser extent.) This allows the language to support very powerful recursive type conversions for parameterized types. Zeolite also allows use-site variance declarations, like Java uses.

Building variance into the core of the type system also allows Zeolite to have a special meta-type that interfaces can use to require that implementations return a value of their own type rather than the type of the interface. This is particularly useful for defining interfaces for iterators and builders, whose methods often perform an update and return a value of the same type.

Parameters as Variables

Zeolite treats type parameters both as type place-holders (like in C++ and Java) and as type variables that you can call functions on. This further allows Zeolite to have interfaces that declare functions that operate on types in addition to interfaces that declare functions that operate on values. (This would be like having abstract static methods in Java.)

This helps solve a few separate problems:

Operations like equals comparisons in Java are always dispatched to the left object, which could lead to inconsistent results if the objects are swapped: foo.equals(bar) is not the same as bar.equals(foo). This dispatching asymmetry can be eliminated by making equals a type function (e.g., MyType.equals(foo, bar)), and further creating an interface that requires implementations to support such calls.
Factory patterns can be abstracted out into interfaces. For example, you could create a factory interface that requires an implementation to parse a new object from a String, without needing to instantiate the factory object itself. You could just implement the factory function directly in MyType, without needing a separate MyTypeFactory.

Integrated Test Runner

The major advantage of statically-typed programming languages is their compile-time detection of code that should not be allowed. On the other hand, there is a major testability gap when it comes to ensuring that your statically-typed code disallows what you expect it to.

Zeolite has a special source-file extension for unit tests, and a built-in compiler mode to run them.

Tests can check for runtime success, compilation success, compilation failure, and even crashes. Normally you would need a third-party test runner to check for required compilation failures and crashes.
The test mode includes a command-line option to collect code-coverage data, which can be critical for determining test efficacy.

Nearly all of the integration testing of the Zeolite language itself is done using this feature, but it is also supported for general use with Zeolite projects.

Integrated Build System

The Zeolite compiler supports a module system that can incrementally compile projects without the user needing to create build scripts or Makefiles.

Modules are configured via a simple config file.
File-level and symbol-level imports and includes are not necessary, allowing module authors to freely rearrange file structure.
Type dependencies are automatically resolved during linking so that output binaries contain only the code that is relevant.
Module authors can back Zeolite code with C++.
The module system is integrated with the compiler's built-in testing mode.

This means that the programmer can focus on code rather than on build rules, and module authors can avoid writing verbose build instructions for the users of their modules.

Data Encapsulation

The overall design of Zeolite revolves around data encapsulation:

No default construction or copying. This means that objects can only be created by explicit factory functions. (A very common mistake in C++ code is forgetting to disallow or override default construction or copying.) This also means that accidental deep copying is not even possible in Zeolite.
Only abstract interfaces can be inherited. Types that define procedures or contain data members cannot be further extended. This encourages the programmer to think more about usage patterns and less about data representation when designing interactions between types.
No "privileged" data-member access. No object has direct access to the data members of any other object; not even other objects of the same type. This forces the programmer to also think about usage patterns when dealing with other objects of the same type.
No null values. Every variable must be explicitly initialized. This obviates questions of whether or not a variable actually contains a value. When combined with the elimination of default construction, this means that a variable can only hold values that a type author has specifically allowed.

Zeolite has an optional storage modifier for use in a variable's type, but it creates a new and incompatible type, whose value cannot be used without first extracting it. This is in contrast to Java allowing any variable to be null. (It is more like Optional in Java 8, but as built-in syntax, and with nesting disallowed.)
No reflection or down-casting. The only thing that has access to the "real" type of a value is the value itself. This means that the reflection and down-casting tricks available in Java to circumvent the compiler are not available in Zeolite. (User-defined types can selectively allow "down-casting like" semantics by reasoning about parameters as variables, however.)
Implementation details are kept separate. In Zeolite, only public inheritance and public functions show up where an object type is declared, to discourage relying on implementation details of the type.

C++ and Java allow (and in some cases require) implementation details (data members, function definitions, etc.) to show up in the same place as the user-accessible parts of a class. The result is that the user of the class will often rely on knowledge of how it works internally.

Although all of these limitations preclude a lot of design decisions allowed in languages like Java, C++, and Python, they also drastically reduce the possible complexity of inter-object interactions. Additionally, they generally do not require ugly work-arounds; see the full examples.

Quick Start

Installation

Requirements:

A POSIX-compliant operating system. Zeolite has been tested on Linux and FreeBSD, and to a limited extent on MacOS. It probably won't work on Windows, due to how it interacts with the filesystem and subprocesses.
A Haskell compiler such as ghc that can install packages using cabal, as well as the cabal installer.
A C++ compiler such as clang++ or g++ and the standard ar archiver present on most Unix-like operating systems.

If you use a modern Linux distribution, most of the above can be installed using the package manager that comes with your distribution. On MacOS, you can install Xcode for a C++ compiler and brew install cabal-install for cabal.

Once you meet all of those requirements, follow the installation instructions for the zeolite-lang package on Hackage. Please take a look at the issues page if you run into problems.

The entire process will probably look like this, once you have cabal and a C++ compiler installed:

$ cabal update
# Also add --overwrite-policy=always if you're upgrading to a specific version.
$ cabal install zeolite-lang
$ zeolite-setup -j8
# Follow interactive prompts...

You might also need to add $HOME/.cabal/bin or $HOME/.local/bin to your $PATH.

For syntax highlighting in Visual Studio Code, See "VS Code Support" in the Zeolite releases and download the .vsix file. If you happen to use the kate text editor, you can use the syntax highlighting in zeolite.xml.

Hello World

It's the any% of programming.

// hello-world.0rx

concrete HelloWorld {
  @type run () -> ()
}

define HelloWorld {
  run () {
    \ BasicOutput.stderr().writeNow("Hello World\n")
  }
}

# Compile.
zeolite -I lib/util --fast HelloWorld hello-world.0rx

# Execute.
./HelloWorld

Also see some full examples for more complete feature usage.

Writing Programs

This section breaks down the separate parts of a Zeolite program. See the full examples for a more integrated language overview.

Basic Ideas

Zeolite programs use object-oriented and procedural programming paradigms. Type categories are used to define object types, much like classes in Java and C++. They are not called "classes", just to avoid confusion about semantic differences with Java and C++.

All type-category names start with an uppercase letter and contain only letters and digits.

All procedures and data live inside concrete type categories. Every program must have at least one concrete category with the procedure to be executed when the program is run.

concrete categories are split into a declaration and a definition. Code for both should be in files ending with .0rx. (The .0rp file type contains only declarations, and will be discussed later.)

// myprogram/myprogram.0rx

// This declares the type.
concrete MyProgram {
  // The entry point must be a () -> () function. This means that it takes no
  // arguments and returns no arguments. (@type will be discussed later.)
  @type run () -> ()
}

// This defines the type.
define MyProgram {
  run () {
    // ...
  }
}

IMPORTANT: All programs or modules must be in their own directory so that zeolite is able to cache information about the build. Unlike some other compilers, you do not specify all command-line options every time you recompile a binary or module.

# Create a new .zeolite-module config. (Only once!)
zeolite -m MyProgram myprogram

# Recompile the module and binary. (After any config or code updates.)
# All sources in myprogram will be compiled. -m MyProgram selects the entry
# point. The default output name for the binary here is myprogram/MyProgram.
zeolite -r myprogram

# Execute.
myprogram/MyProgram

# An alternative, if you only have one .0rx and want to quickly iterate.
zeolite --fast MyProgram myprogram/myprogram.0rx

Declaring Functions

A function declaration specifies the scope of the function and its argument and return types. (And optionally type parameters and parameter filters, to be discussed later.) The declaration simply indicates the existence of a function, without specifying its behavior.

All function names start with a lowercase letter and contain only letters and digits.

concrete MyCategory {
  // @value indicates that this function requires a value of type MyCategory.
  // This function takes 2x Int and returns 2x Int.
  @value minMax (Int, Int) -> (Int, Int)

  // @type indicates that this function operates on MyCategory itself. This is
  // like a static function in C++.
  // This function takes no arguments and returns MyCategory.
  @type create () -> (MyCategory)

  // @category indicates that this function operates on MyCategory itself. This
  // is like a static function in Java. (The semantics of @category are similar
  // to those of @type unless there are type parameters.)
  @category copy (MyCategory) -> (MyCategory)
}

In many cases, the choice between using a @category function or a @type function is arbitrary, but there are pros and cons of each:

@category functions do not inherit any of the category's parameters, or their filters. This can be useful in a few situations:
1. You want to impose additional restrictions on what parameters can be used. For example, Vector:createSize (lib/container) requires that param #y defines Default so that it can populate the Vector with default values.
2. The caller will pass arguments that can be used to infer the category's parameters. For example, Vector:duplicateSize (lib/container) takes a single #y. Since #y is a function param, it can be inferred from the argument that gets passed, e.g., Vector:duplicateSize(0, 25).
If neither of these situations apply, a @type function might be better.
@type functions do inherit the category's parameters and their filters, which means that they do not need to be specified again, and they do not need to be passed again when calling from another @type or @value function. This is more efficient to maintain and execute.

@type interfaces are another advantage of @type functions.

Defining Functions

Functions are defined in the category definition. They do not need to repeat the function declaration; however, they can do so in order to refine the argument and return types for internal use.

All function names start with a lowercase letter and contain only letters and digits.

The category definition can also declare additional functions that are not visible externally.

concrete MyCategory {
  @type minMax (Int, Int) -> (Int, Int)
}

define MyCategory {
  // minMax is defined here.
  minMax (x, y) {
    if (superfluousCheck(x, y)) {
      return x, y
    } else {
      return y, x
    }
  }

  // superfluousCheck is only available inside of MyCategory.
  @type superfluousCheck (Int, Int) -> (Bool)
  superfluousCheck (x, y) {
    return x < y
  }
}

All arguments must either have a unique name or be ignored with _.

@value functions have access to a special constant self, which refers to the object against which the function was called.

Using Variables

Variables are assigned with <- to indicate the direction of assignment. Every variable must be initialized; there are no null values in Zeolite. (However, see optional later on.)

All variable names start with a lowercase letter and contain only letters and digits.

When a location is needed for assignment (e.g., handling a function return, taking a function argument), you can use _ in place of a variable name to ignore the value.

// Initialize with a literal.
Int value <- 0

// Initialize with a function result.
Int value <- getValue()

Unlike other languages, Zeolite does not allow variable masking. For example, if there is already a variable named x available, you cannot create a new x variable even in a smaller scope.

All variables are shared and their values are not scoped like they are in C++. You should not count on knowing the lifetime of any given value.

As of compiler version 0.24.0.0, you can also swap the values of two variables that have the same type, as long as both are writable. This is more efficient than "manually" swapping using a temp variable.

Int foo <- 123
Int bar <- 456
foo <-> bar

Calling Functions

Return values from function calls must always be explicitly handled by assigning them to a variable, passing them to another function or ignoring them. (This is required even if the function does not return anything, primarily to simplify parsing.)

// Utilize the return.
Int value <- getValue()

// Explicitly ignore a single value.
_ <- getValue()

// Ignore all aspects of the return.
// (Prior to compiler version 0.3.0.0, ~ was used instead of \.)
\ printHelp()

Calling a function with @value scope requires a value of the correct type, and uses . notation, e.g., foo.getValue().
Calling a function with @type scope requires the type with parameter substitution (if applicable), and uses . notation, e.g., MyCategory<Int>.create(). (Prior to compiler version 0.9.0.0, $ was used instead of ..)
Calling a function with @category scope requires the category itself, and uses the : notation, e.g., MyCategory:foo(). (Prior to compiler version 0.9.0.0, $$ was used instead of :.)
You can skip qualifying function calls (e.g., in the example above) if the function being called is in the same scope or higher. For example, you can call a @type function from the procedure for a @value function in the same category.

Functions cannot be overloaded like in Java and C++. Every function must have a unique name. Functions inherited from different places can be explicitly merged, however. This can be useful if you want interfaces to have overlapping functionality without having an explicit parent for the overlap.

@value interface ForwardIterator<|#x> {
  next () -> (optional #self)
  get () -> (#x)
}

@value interface ReverseIterator<|#x> {
  prev () -> (optional #self)
  get () -> (#x)
}

concrete Iterator<|#x> {
  refines ForwardIterator<#x>
  refines ReverseIterator<#x>

  // An explicit override is required in order to merge get from both parents.
  get () -> (#x)
}

Functions As Operators

Zeolite allows some functions to be used as operators. This allows users to avoid excessive parentheses when using named mathematical functions.

Functions with two arguments can use infix notation. The operator precedence is always between comparisons (e.g., ==) and logical (e.g., &&).

Functions with one argument can use prefix notation. These are evaluated strictly before all infix operators.

concrete Math {
  @type plus (Int, Int) -> (Int)
  @type neg (Int) -> (Int)
}

// ...

// Math.plus is evaluated first.
Int x <- 1 `Math.plus` 2 * 5
// Math.neg is evaluated first.
Int y <- `Math.neg` x `Math.plus` 2

Data Members and Value Creation

Unlike Java and C++, there is no "default construction" in Zeolite. In addition, Zeolite also lacks the concept of "copy construction" that C++ has. This means that new values can only be created using a factory function. In combination with required variable initialization, this ensures that the programmer never needs to worry about unexpected missing or uninitialized values.

Data members are never externally visible; they only exist in the category definition. Any access outside of the category must be done using explicitly-defined functions.

concrete MyCategory {
  @type create () -> (MyCategory)
}

define MyCategory {
  // A data member unique to each MyCategory value.
  @value Int value

  create () {
    // Initialization is done with direct assignment.
    return MyCategory{ 0 }
  }
}

// ...

// Create a new value in some other procedure.
MyCategory myValue <- MyCategory.create()

There is no syntax for accessing a data member from another object; even objects of the same type. This effectively makes all variables internal rather than just private like in Java and C++. As long as parameter variance is respected, you can provide access to an individual member with getters and setters.

As of compiler version 0.14.0.0, you can use #self in place of the full type when you are creating a value of the same type from a @type or @value function.

concrete MyCategory {
  @type create () -> (MyCategory)
}

define MyCategory {
  @value Int value

  create () {
    return #self{ 0 }
  }
}

A category can also have @category members, but not @type members. (The latter is so that the runtime implementation can clean up unused @types without introducing ambiguitites regarding member lifespan.)

concrete MyCategory {
  @type global () -> (MyCategory)
}

define MyCategory {
  @value Int value

  // @category members use inline initialization.
  @category MyCategory singleton <- MyCategory{ 0 }

  global () {
    // @category members are accessible from all functions in the category.
    return singleton
  }
}

Conditionals

Zeolite uses the if/elif/else conditional construct. The elif and else clauses are always optional.

if (x) {
  // something
} elif (y) {
  // something
} else {
  // something
}

Scoping and Cleanup

Variables can be scoped to specific blocks of code. Additionally, you can provide a cleanup procedure to be executed upon exit from the block of code. This is useful if you want to free resources without needing to explicitly do so for every return statement.

// Simple scoping during evaluation.
scoped {
  Int x <- getValue()
} in if (x < 0) {
  // ...
} elif (x > 0) {
  // ...
} else {
  // ...
}

// Simple scoping during assignment.
scoped {
  Int x <- getValue1()
  Int y <- getValue2()
} in Int z <- x+y

// Scoping with cleanup.
scoped {
  // ...
} cleanup {
  // ...
} in {
  // ...
}

// Cleanup without scoping.
cleanup {
  i <- i+1  // Post-increment behavior.
} in return i

The cleanup block is executed at every return, break, and continue in the respective in block, and right after the in block. For this reason, you cannot use return, break, or continue within a cleanup block. Additionally, you cannot overwrite named returns. You can use fail, however, since that just ends program execution.

When cleanup is executed at a return statement in the in block, the returns from the return statement are "locked in", then cleanup is executed, then those locked-in return values are returned. (This is what allows the post-increment example above to work.)

Loops

Zeolite supports two loop types:

while loops, which are the traditional repetition of a procedure while a predicate holds.

// With break and continue.
while (true) {
  if (true) {
    break
  } else {
    continue
  }
}

// With an update after each iteration.
while (true) {
  // ...
} update {
  // ...
}

traverse loops (as of compiler version 0.16.0.0), which automatically iterate over the #x values in an optional Order<#x>. This is similar to for (int i : container) { ... } in C++ and for i in container: ... in Python.
```
traverse (orderedStrings -> String s) {
  // executed once per String s in orderedStrings
  // you can also use break and continue
}

// With an update after each iteration.
traverse (orderedStrings -> String s) {
  // ...
} update {
  // ...
}
```
Since the Order is optional, empty can be used to iterate zero times.

IMPORTANT: Most containers are not iterable by traverse as-is; you will need to call a @value function to get the Order. Some categories refine DefaultOrder<#x> (such as String, and Vector in lib/container), which allows you to use its defaultOrder(). Other categories provide multiple ways to Order the container, such as SearchTree in lib/container.
```
traverse ("hello".defaultOrder() -> Char c) {
  // executed once per Char c in "hello"
}
```

for loops (e.g., for (int i = 0; i < foo; ++i) { ... } in C++) are not supported, since such syntax is too restrictive to scale, and they can be replaced with traverse or scoped+while in nearly all situations.

// Combine while with scoped to create a for loop.
scoped {
  Int i <- 0
  Int limit <- 10
} in while (i < limit) {
  // ...
} update {
  i <- i+1
}

Multiple Returns

A procedure definition has two options for returning multiple values:

Return all values. (Prior to compiler version 0.3.0.0, multiple returns were enclosed in {}, e.g., return { x, y }.)

define MyCategory {
  minMax (x, y) {
    if (x < y) {
      return x, y
    } else {
      return y, x
    }
  }
}

Naming the return values and assigning them individually. This can be useful (and less error-prone) if the values are determined at different times. The compiler uses static analysis to ensure that all named variables are guaranteed to be set via all possible control paths.

define MyCategory {
  // Returns are named on the first line.
  minMax (x, y) (min, max) {
    // Returns are optionally initialized up front.
    min <- y
    max <- x
    if (x < y) {
      // Returns are overwritten.
      min <- x
      max <- y
    }
    // Implicit return makes sure that all returns are assigned. Optionally,
    // you can use return _.
  }
}

To return early when using named returns or when the function has no returns, use return _. You will get an error if a named return might not be set.

The caller of a function with multiple returns also has a few options:

Assign the returns to a set of variables. You can ignore a position by using _ in that position. (Prior to compiler version 0.3.0.0, multiple assignments were enclosed in {}, e.g., { Int min, _ } <- minMax(4,3).)
```
Int min, _ <- minMax(4, 3)
```
Pass them directly to a function that requires the same number of compatible arguments. (Note that you cannot concatenate the returns of multiple functions.)
```
Int delta <- diff(minMax(4, 3))
```
If you need to immediately perform an operation on just one of the returned values while ignoring the others, you can select just that return inline. (As of compiler version 0.21.0.0.)
```
// Select return 0 from minMax.
return minMax(4, 3){0}
```
Note that the position must be an integer literal so that the compiler can validate both the position and the return type.

Optional and Weak Values

Zeolite requires that all variables be initialized; however, it provides the optional storage modifier to allow a specific variable to be empty. This is not the same as null in Java because optional variables need to be required before use.

// empty is a special value for use with optional.
optional Int value <- empty

// Non-optional values automatically convert to optional.
value <- 1

// present returns true iff the value is not empty.
if (present(value)) {
  // Use require to convert the value to something usable.
  \ foo(require(value))
}

As of compiler version 0.24.0.0, you can use <-| to conditionally overwrite an optional variable if it's currently empty.

optional Int value <- empty
value <-| 123    // Assigned, because value was empty.
value <-| 456    // Not assigned, because value wasn't empty.
value <-| foo()  // foo() isn't called unless value is empty.

Note that if the right side isn't optional then you can use the result as non-optional.

optional Int value <- empty
Int value2 <- (value <-| 123)

As of compiler version 0.24.0.0, you can conditionally call a function on an optional value if it's non-empty using &..

optional Int value <- 123
// All returned values will be optional.
optional Formatted formatted <- value&.formatted()
// foo() won't be called unless the readAt call is going to be made.
optional Char char <- formatted&.readAt(foo())

As of compiler version 0.24.1.0, you can use x <|| y to use y if x is empty. Note that x must have an optional type, and the resulting type of the entire expression is the type union of the types of x and y.

weak values allow your program to access a value if it is available, without holding up that value's cleanup if nothing else needs it. This can be used to let threads clean themselves up (example below) or to handle cycles in references between objects.

concrete MyRoutine {
  @type createAndRun () -> (MyRoutine)
  @value waitCompletion () -> ()
}

define MyRoutine {
  refines Routine

  // (See lib/thread.)
  @value weak Thread thread

  createAndRun () {
    // Create a new MyRoutine and then start the thread.
    return MyRoutine{ empty }.start()
  }

  run () {
    // routine
  }

  waitCompletion () {
    scoped {
      // Use strong to turn weak into optional. If the return is non-empty, the
      // value is guaranteed to remain valid while using thread2.
      optional Thread thread2 <- strong(thread)
    } in if (present(thread2)) {
      \ require(thread2).join()
    }
  }

  @value start () -> (#self)
  start () {
    // ProcessThread holds a reference to itself only while the Routine is
    // running. Making thread weak means that the ProcessThread can clean itself
    // up once the Routine terminates.
    thread <- ProcessThread.from(self).start()
    return self
  }
}

Deferred Variable Initialization

In some situations, a variable's value depends on conditional logic, and there is no low-cost default value. In such situations, you can use the defer keyword to allow a variable to be temporarily uninitialized. (As of compiler version 0.20.0.0.)

LargeObject object <- defer

if (debug) {
  object <- LargeObject.newDebug()
} else {
  object <- LargeObject.new()
}

\ object.execute()

In this example, object is declared without an initializer, and is then initialized in both the if and else clauses.

A variable initialized with defer must be initialized via all possible control paths prior to its use. This is checked at compile time.
An existing variable can also be marked as deferred. This will not change its value, but will instead require that it be assigned a new value before it gets used again.
If you never read the variable in a particular control branch then you do not need to initialize it; initialization is only checked where necessary.

Immediate Program Termination

There are two ways to terminate the program immediately.

The fail builtin can be used to immediately terminate the program with a stack trace. This is not considered a function call since it cannot return; therefore, do not precede it with \.
```
define MyProgram {
  run () {
    fail("MyProgram does nothing")
  }
}
```
The value passed to fail must implement the Formatted builtin @value interface.

The output to stderr will look something like this:
```
./MyProgram: Failed condition: MyProgram does nothing
  From MyProgram.run at line 7 column 5 of myprogram.0rx
  From main
Terminated
```
The exit builtin can be used to immediately terminate the program with a traditional Int exit code. (0 conventionally means program success.) This is not considered a function call since it cannot return; therefore, do not precede it with \.
```
define MyProgram {
  run () {
    exit(0)
  }
}
```
The value passed to exit must be an Int.

Call Delegation

As of compiler version 0.24.0.0, you can delegate function calls using the delegate keyword. This has the effect of forwarding all of the arguments passed to the enclosing function call to the handler specified. (The call is actually rewritten using a substitution during compilation.)

new (value1,value2) {
  // Same as Value{ value1, value2 }.
  \ delegate -> Value

  // Same as foo(value1,value2).
  \ delegate -> `foo`

  // Same as something(123).bar(value1,value2).
  \ delegate -> `something(123).bar`
}

IMPORTANT: If the enclosing function specifies argument labels then those will be used in the forwarded call.

@type new (String name:, Int) -> (Value)
new (value1,value2) {
  // Same as foo(name: value1, value2).
  return delegate -> `foo`
}

IMPORTANT: Delegation will fail to compile if:

One or more function arguments is ignored with _, e.g., call(_) { ... }.
One or more function arguments is hidden with $Hidden[]$ , e.g., $Hidden[someArg]$ .

This is primarily as a sanity check, since all of the above imply that a given argument should not be used.

Function Argument Labels

As of compiler version 0.24.0.0, function declarations in Zeolite can optionally have labels for any individual argument. Note that this is a label and not an argument name.

All labels start with a lowercase letter and contain only letters and digits, and end with :.

The syntax for labeling an argument in the function declaration is to specify it after the type.

concrete Value {
  // The first argument requires start: as a label.
  @type new (Int start:) -> (Value)
}

The syntax for labeling an argument in the function call is to precede the argument with the label.
```
// The first argument is labeled with start:.
Value foo <- Value.new(start: 123)
```
IMPORTANT: If the function declaration specifies a label, the label must always be used when calling that function. Additionally, arguments must still be passed in the same order; labels don't allow you to reorder arguments.

When defining a function, the name you give to an argument should match the label, but that isn't a requirement. Also note that labels can be reused, e.g., @value swapRows (Int row:, Int row:) -> (). This allows the label to be descriptive rather than just an identifier.

Using Parameters

All concrete categories and all interfaces can have type parameters. Each parameter can have a variance rule assigned to it. This allows the compiler to do type conversions between different parameterizations.

Parameter names must start with # and a lowercase letter, and can only contain letters and digits.

Parameters are never repeated in the category or function definitions. (Doing so would just create more opportunity for unnecessary compile-time errors.)

// #x is covariant (indicated by being to the right of |), which means that it
// can only be used for output purposes.
@value interface Reader<|#x> {
  read () -> (#x)
}

// #x is contravariant (indicated by being to the left of |), which means that
// it can only be used for input purposes.
@value interface Writer<#x|> {
  write (#x) -> ()
}

// #x is for output and #y is for input, from the caller's perspective.
@value interface Function<#x|#y> {
  call (#x) -> (#y)
}

// By default, parameters are invariant, i.e., cannot be converted. You can also
// explicitly specify invariance with <|#x|>. This allows all three variance
// types to be present.
concrete List<#x> {
  @value append (#x) -> ()
  @value head () -> (#x)
}

// Use , to separate multiple parameters that have the same variance.
concrete KeyValue<#k, #v> {
  @type new (#k, #v) -> (#self)
  @value key   () -> (#k)
  @value value () -> (#v)
}

Specifying parameter variance allows the compiler to automatically convert between different types. This is done recursively in terms of parameter substitution.

// Covariance allows conversion upward.
Reader<MyValue> reader <- // ...
Reader<MyBase>  reader2 <- reader

// Contravariance allows conversion downward.
Writer<MyBase>  writer <- // ...
Writer<MyValue> writer2 <- writer

// Conversion is also recursive.
Writer<Reader<MyBase>>  readerWriter <- // ...
Writer<Reader<MyValue>> readerWriter2 <- readerWriter

// Invariance does not allow conversions.
List<MyValue> list <- // ...
List<MyBase>  list2 <- // ...

You can apply filters to type parameters to require that the parameters meet certain requirements. This allows you to call interface functions on parameters and their values in procedure definitions.

concrete Helper {
  @type format<#x>
    // Ensures that #x -> Formatted. (Like T extends Foo in Java.)
    // Example: String f <- Helper.format(123)
    #x requires Formatted
  (#x) -> (String)

  @type get<#x>
    // Ensures that #x <- String. (Like T super Foo in Java.)
    // Example: AsBool v <- Helper.get<AsBool>()
    #x allows String
  () -> (#x)

  @type create<#x>
    // Ensures that #x defines the Default @type interface.
    // Example: Int v <- Helper.create<Int>()
    #x defines Default
  () -> (#x)
}

define Helper {
  format (x) {
    // #x -> Formatted means x has formatted().
    return x.formatted()
  }

  get () {
    // #x <- String means we can return a String here.
    return "message"
  }

  create () {
    // #x defines Default means #x has default().
    return #x.default()
  }
}

Filters on category params must be specified after all refines/defines and before any function declarations.

IMPORTANT: As of compiler version 0.16.0.0, you can no longer use parameter filters in @value interfaces and @type interfaces.

Using Interfaces

Zeolite has @value interfaces that are similar to Java interfaces, which declare functions that implementations must define. In addition, Zeolite also has @type interfaces that declare @type functions that must be defined. (This would be like having abstract static functions in Java.)

// @value indicates that the interface declares @value functions.
@value interface Printable {
  // @value is not allowed in the declaration.
  print () -> ()
}

// @type indicates that the interface declares @type functions.
@type interface Diffable<#x> {
  // @type is not allowed in the declaration.
  diff (#x, #x) -> (#x)
}

Type	Param Variance	Param Filters	Can Inherit	`@category` Funcs	`@type` Funcs	`@value` Funcs	Define Procedures
`concrete`	✓	✓	`@value interface` `@type interface`	✓	✓	✓	✓
`@value interface`	✓		`@value interface`			✓
`@type interface`	✓		--		✓

@value interfaces can be inherited by other @value interfaces and concrete categories using refines.

concrete MyValue {
  refines Printable

  @type new (Int) -> (MyValue)

  // The functions of Printable do not need to be declared again, but you can do
  // so to refine the argument and return types.
}

define MyValue {
  @value Int value

  new (value) {
    return MyValue{ value }
  }

  // Define Printable.print like any other MyValue function.
  print () {
    \ BasicOutput.writeNow(value)
  }
}

@type interfaces can only be inherited by concrete categories.

concrete MyValue {
  defines Diffable<MyValue>

  @type new (Int) -> (MyValue)

  // The functions of Diffable do not need to be declared again, but you can do
  // so to refine the argument and return types.
}

define MyValue {
  @value Int value

  new (value) {
    return MyValue{ value }
  }

  // Define Diffable.diff like any other MyValue function.
  diff (x, y) {
    return MyValue{ x.get() - y.get() }
  }

  // A getter is needed to access the value outside of the object that owns it.
  @value get () -> (Int)
  get () {
    return value
  }
}

You can also specify refines and defines when defining a concrete category. This allows the inheritance to be private.

concrete MyValue {
  @type create () -> (Formatted)
}

define MyValue {
  // Formatted is not a visible parent outside of MyValue.
  refines Formatted

  create () {
    return MyValue{ }
  }

  // Inherited from Formatted.
  formatted () {
    return "MyValue"
  }
}

Immutable Types

You can modify interface and concrete with immutable at the very top of the declaration. (As of compiler version 0.20.0.0.) This creates two requirements for @value members:

They are marked as read-only, and cannot be overwritten with <-.
They must have a type that is also immutable.

(@category members are not affected.)

Note that this applies to the entire implementation; not just to the implementations of functions required by the immutable interface. immutable is therefore intended for objects that cannot be modified, rather than as a way to define a read-only view (e.g., const in C++) of an object.

@value interface Foo {
  immutable

  call () -> ()
}

concrete Bar {
  refines Foo

  @type new () -> (Bar)
  @value mutate () -> ()
}

define Bar {
  @value Int value

  new () { return Bar{ 0 } }

  call () {
    // call cannot overwrite value
  }

  mutate () {
    // mutate also cannot overwrite value, even though mutate isn't in Foo.
  }
}

For members that use a parameter as a type, you can use immutable as a filter if the other filters do not otherwise imply it. Note that this will prevent substituting in a non-immutable type when calling @type functions.

concrete Type<#x> {
  immutable

  #x immutable
}

define Type {
  // #x is allowed as a member type because of the immutable filter.
  @value #x value
}

The `#self` Parameter

Every category has an implicit covariant parameter #self. (As of compiler version 0.14.0.0.) It always means the type of the current category, even when inherited. (#self is covariant because it needs to be convertible to a parent of the current category.)

For example:

@value interface Iterator<|#x> {
  next () -> (#self)
  get () -> (#x)
}

concrete CharIterator {
  refines Iterator<Char>
  // next must return CharIterator because #self = CharIterator here.
}

The primary purpose of this is to support combining multiple interfaces with iterator or builder semantics into composite types without getting backed into a corner when calling functions from a single interface.

@value interface ForwardIterator<|#x> {
  next () -> (#self)
  get () -> (#x)
}

@value interface ReverseIterator<|#x> {
  prev () -> (#self)
  get () -> (#x)
}

concrete CharIterator {
  refines ForwardIterator<Char>
  refines ReverseIterator<Char>
  get () -> (Char)  // (Remember that merging needs to be done explicitly.)
}

concrete Parser {
  // trimWhitespace can call next and still return the original type. In
  // contrast, if next returned ForwardIterator<#x> then trimWhitespace would
  // need to return ForwardIterator<Char> to the caller instead of #i.
  @type trimWhitespace<#i>
    #i requires ForwardIterator<Char>
  (#i) -> (#i)
}

#self can also be used to generalize a factory pattern:

@type interface ParseFactory {
  fromString (String) -> (#self)
}

concrete FileParser {
  @type parseFromFile<#x>
    #x defines ParseFactory
  (String) -> (#x)
}

define FileParser {
  parseFromFile (filename) {
    String content <- FileHelper.readAll(filename)
    // Notice that ParseFactory doesn't need a type parameter to indicate what
    // type is going to be parsed in fromString; it's sufficient to know that #x
    // implements ParseFactory and that fromString returns #self.
    return #x.fromString(content)
  }
}

concrete Value {
  defines ParseFactory
}

define Value {
  fromString (string) {
    if (string == "Value") {
      return Value{ }
    } else {
      fail("could not parse input")
    }
  }
}

#self is nothing magical; this could all be done by explicitly adding a covariant #self parameter to every type, with the appropriate requires and defines filters.

Type Inference

Starting with compiler version 0.7.0.0, Zeolite supports optional inference of specific function parameters by using ?. This must be at the top level (no nesting), and it cannot be used outside of the parameters of the function.

The type-inference system is intentionally "just clever enough" to do things that the programmer can easily guess. More sophisticated inference is feasible in theory (like Haskell uses); however, type errors with such systems can draw a significant amount of attention away from the task at hand. (For example, a common issue with Haskell is not knowing which line of code contains the actual mistake causing a type error.)

concrete Value<#x> {
  @category create1<#x> (#x) -> (Value<#x>)
  @type     create2     (#x) -> (Value<#x>)
}

// ...

// This is fine.
Value<Int> value1 <- Value:create1<?>(10)

// These uses of ? are not allowed:
// Value<Int> value2 <- Value<?>.create2(10)
// Value<?>   value2 <- Value<Int>.create2(10)

Only the function arguments and the parameter filters are used to infer the type substitution; return types are ignored. If inference fails, you will see a compiler error and will need to explicitly write out the type.

As of compiler version 0.21.0.0, if you want to infer all params, you can skip <...> entirely. If you only want to infer some of the params, you must specify all params, using ? for those that should be inferred.

Type inference will only succeed if:

There is a valid pattern match between the expected argument types and the types of the passed arguments.
There is exactly one type that matches best:
- For params only used in covariant positions, the lower bound of the type is unambiguous.
- For params only used in contravariant positions, the upper bound of the type is unambiguous.
- For all other situations, the upper and lower bounds are unambiguous and equal to each other.

Type inference in the context of parameterized types is specifically disallowed in order to limit the amount of code the reader needs to search to figure out what types are being used. Forcing explicit specification of types for local variables is more work for the programmer, but it makes the code easier to reason about later on.

Other Features

This section discusses language features that are less frequently used.

Meta Types

Zeolite provides two meta types that allow unnamed combinations of other types.

A value with an intersection type [A & B] can be assigned from something that is both A and B, and can be assigned to either an A or B. There is a special empty intersection named any that can be assigned from any value but cannot be assigned to any other type.

Intersections can be useful for requiring multiple interfaces without creating a new category that refines all of those interfaces. An intersection [Foo & Bar] in Zeolite is semantically similar to the existential type forall a. (Foo a, Bar a) => a in Haskell and ? extends Foo & Bar in Java, except that in Zeolite [Foo & Bar] can be used as a first-class type.
```
@value interface Reader { }

@value interface Writer { }

concrete Data {
  refines Reader
  refines Writer
  @type new () -> (Data)
}

// ...

[Reader & Writer] val <- Data.new()
Reader val2 <- val
Writer val3 <- val
```

A value with a union type [A | B] can be assigned from either A or B, but can only be assigned to something that both A and B can be assigned to. There is a special empty union named all that cannot ever be assigned a value but that can be assigned to everything. (empty is actually of type optional all.)

Unions can be useful if you want to artificially limit what types can be used in a particular context. This can be useful for disallowing use of "unknown" implementations in critical or risky functions, etc.

When used with requires, a union can limit the allowed types without losing the original type.

concrete Helper {
  @type describe<#x>
    #x requires Formatted
    #x requires [String | Int]  // <- limits allowed types
  (#x) -> (String)
}

define Helper {
  describe (value) {
    return String.builder()
        .append(typename<#x>())  // <- original type is still available
        .append(": ")
        .append(value)
        .build()
  }
}

// ...

\ Helper.describe(123)        // Fine.
\ Helper.describe("message")  // Fine.
\ Helper.describe(123.0)      // Error!

When used as a variable type, the original type is lost, but you can still convert to a common parent type.

[String | Int] value <- 123
// You can only convert to types that _all_ constituent types convert to.
Formatted formatted <- value  // Fine.
Int number <- value           // Error!

Intersection and union types also come up in type inference.

If there are two or more incompatible guesses for an inferred type used only for input, the union of those types will be used.
If there are two or more incompatible guesses for an inferred type used only for output, the intersection of those types will be used.

// (Just for creating an output parameter.)
concrete Writer<#x|> {
  @type new () -> (#self)
}

concrete Helper {
  // #x is only used for input to the function.
  @type inferInput<#x> (#x, #x) -> (String)

  // #x is only used for output from the function.
  // (This is due to contravariance of #x in Writer.)
  @type inferOutput<#x> (Writer<#x>, Writer<#x>) -> (String)
}

define Helper {
  inferInput (_, _) { return typename<#x>().formatted() }
  inferOutput (_, _) { return typename<#x>().formatted() }
}

define Writer {
  new () { return #self{ } }
}

// ...

// Returns "[Int | String]".
\ Helper.inferInput(123, "message")

// Returns "[Int & String]".
\ Helper.inferOutput(Writer<Int>.new(), Writer<String>.new())

In this context, unions/intersections are the most restrictive valid types that will work for the substution. (They are respectively the coproduct/product of the provided types under implicit type conversion.)

Explicit Type Conversion

In some situations, you might want to peform an explicit type conversion on a @value. The syntax for such conversions is value?Type, where value is any @value and Type is any type, including params and meta types.

With values that have a union type (e.g., [A | B]), you might need an explicit type conversion when making a function call.

@value interface Object<|#x> {
  get () -> (#x)
}

concrete IntObject {
  refines Object<Int>
  @type new (Int) -> (IntObject)
}

concrete StringObject {
  refines Object<String>
  @type new (String) -> (StringObject)
}

// ...

[IntObject | StringObject] value <- StringObject.new("message")

// Convert to Object<Formatted> before calling get().
Formatted formatted <- value?Object<Formatted>.get()

// Should get() return Int or String here?
// Formatted formatted <- value.get()

Type conversions of function arguments can be used for influencing type inference.

concrete Helper {
  @category call<#x> (#x) -> ()
}

// ...

Int value <- 1

// #x will be inferred as Formatted rather than as Int here.
\ Helper:call(value?Formatted)

You can also explicitly convert optional and weak values, although they will still retain their original storage modifier.

optional Int value <- 1

// Passed as optional Formatted.
\ call(value?Formatted)

// Not allowed, since value.Formatted is still optional.
// String string <- value?Formatted.formatted()

Runtime Type Reduction

The reduce builtin function enables very limited runtime reasoning about type conversion.

The call reduce<Foo, Bar>(value) will return value with type optional Bar iff Foo can be converted to Bar. Note that value must itself be convertible to optional Foo.
When type params are used, the types that are assigned at the point of execution are checked. For example, the result of reduce<#x, #y>(value) will depend on the specific types assigned to #x and #y upon execution.

Here are a few motivating use-cases:

Allowing creation of a container that can hold objects of different types while still being able to access the objects with their original types. (Also see TypeMap in lib/container, which was actually the original target use-case for the initial version of Zeolite.)
Enabling optional functionality for a parameter without using a filter. For example, printing info about value if available using reduce<#x, Formatted>(value) during debugging.

@value interface AnyObject {
  getAs<#y> () -> (optional #y)
}

concrete Object<#x> {
  @category create<#x> (#x) -> (AnyObject)
}

define Object {
  refines AnyObject

  @value #x value

  create (value) { return Object<#x>{ value } }
  getAs  ()      { return reduce<#x, #y>(value) }
}

AnyObject value <- Object:create<?>("message")

// This will be empty because String does not convert to Int.
optional Int value1 <- value.getAs<Int>()

// This will be "message" as Formatted because String converts to Formatted.
optional Formatted value2 <- value.getAs<Formatted>()

reduce cannot be used to "downcast" a value (e.g., converting a Formatted to a Float) since the argument has the same type as the first parameter.

For example, reduce<#x, #y>(value) checks #x→#y, and since value must be optional #x, value can only be converted upward. In other words, it only allows conversions that would otherwise be allowed, returning empty for all other conversions.

The AnyObject example above works because Object stores the original type passed to create as #x, which it then has available for the reduce call. The type variables #x and #y are the primary inputs to reduce; there is absolutely no examination of the "real" type of value at runtime.

// Here we explicitly set #x = Formatted when calling create.
AnyObject value <- Object:create<Formatted>("message")

// This will be empty even though the actual value is a String because getAs
// uses #x = Formatted in the reduce call.
optional String value1 <- value.getAs<String>()

Value Instance Comparison

As of compiler version 0.24.0.0, you can get a value that identifies a specific @value instance using the identify builtin.. This can be useful for creating identifiers that don't otherwise have a unique member.

String value <- "value"
Identifier<String> valueId <- identify(value)

You can use comparison operators (e.g., ==, <) between Identifiers of any two types.
The Identifier remains valid even if the original @value is deallocated.
identify can also be used for optional types, but not for weak types.

Type conversions have no effect on the resulting Identifier, other than the type used for compile-time checking.

String value <- "value"
// The following are equivalent:
Identifier<Formatted> id1 <- identify(value)
Identifier<Formatted> id2 <- identify(value?Formatted)

Identifier uniqueness isn't reliable for unboxed types (e.g., Int) because the values are stored without being contained in a Zeolite object. (Also see Builtin Types.) For example, identifier(2) == identifier(2), whereas identifier("value") != identifier("value"), due to storage differences.

Limited Function Visibility

As of compiler version 0.24.0.0, you can restrict where @value and @type functions can be called from with the visibility keyword.

concrete Value {
  // This applies to everything below.
  visibility Factory

  // This can only be called from Factory.
  @type new () -> (Value)

  // This resets the visibility to the default.
  visibility _

  // This can be called from anywhere.
  @value call () -> ()
}

You can specify multiple types separated by ,.
You will get a compiler error if you declare a @category function when the visibility is other than the default.
Functions with restricted visibility can't be called from @category functions because the full type where the call originates isn't defined.
The category's own definition and unittests are exempt from visibility requirements.
visibility all will prevent functions from being called outside of the category itself and unittest.
visibility any will prevent functions from getting called in @category functions but not anywhere else.
For @value functions with limited visibility, sometimes an explicit type conversion of the value will be needed prior to making the function call, e.g., value?Parent.call(). The effect is that the allowed argument and return types could be more limited.

Builtins

Reserved Words

#self @category @type @value _ all allows any break cleanup concrete continue defer define defines delegate elif else empty exit fail false identify if immutable in interface optional present reduce refines require requires return scoped self strong testcase traverse true typename unittest update visibility weak while

Builtin Types

See builtin.0rp and testing.0rp for more details about builtin types. (For your locally-installed version, which might differ, see $(zeolite --get-path)/base/builtin.0rp.)

Builtin concrete types:

Bool [unboxed]: Either true or false.
Char [unboxed]: Use single quotes, e.g., 'a'. Use literal characters, standard escapes (e.g., '\n'), 2-digit hex (e.g., \x0B), or 3-digit octal (e.g., '\012'). At the moment this only supports ASCII; see Issue #22.
CharBuffer: Mutable, fixed-size buffer of Char. This type has no literals.
Float [unboxed]: Use decimal notation, e.g., 0.0 or 1.0E1. You must have digits on both sides of the .. As of compiler version 0.24.0.0, you can also escape hex with \x, octal with \o, and binary with \b.
Identifier<#x> [unboxed]: An opaque identifier used to compare underlying @value instances.
Int [unboxed]: Use decimal (e.g., 1234), hex (e.g., \xABCD), octal (e.g., \o0123), or binary (e.g., \b0100).
Pointer<#x> [unboxed]: An opaque pointer type for use in C++ extensions. Only C++ extensions can create and access Pointer contents, but Zeolite code can still store and pass them around.
String: Use double quotes to sequence Char literals, e.g., "hello\012". You can build a string efficiently using String.builder(), e.g., String foo <- String.builder().append("bar").append("baz").build().

Builtin @value interfaces:

Append<#x>: Supports appending #x.
AsBool: Convert a value to Bool using asBool().
AsChar: Convert a value to Char using asChar().
AsFloat: Convert a value to Float using asFloat().
AsInt: Convert a value to Int using asInt().
Build<#x>: Build a #x from the current state.
Container: Contains multiple values.
DefaultOrder<#x>: The container provides a default Order<#x> for use with traverse.
Duplicate: Duplicate the value with duplicate().
Formatted: Format the value as a String using formatted().
Hashed: Hash the value as an Int using hashed().
Order<#x>: An ordering of #x values, for use with the traverse
ReadAt<#x>: Random access reads from a container with values of type #x.
SubSequence: Extract a subsequence from the object.
WriteAt<#x>: Random access writes to a container with values of type #x.

Builtin @type interfaces:

Default: Get the default @value with default().
Equals<#x>: Compare values using equals(x,y).
LessThan<#x>: Compare values using lessThan(x,y).
Testcase: For use in testcase.

Builtin meta-types:

#self: The type of the category where it is used. See #self.
any: Value type that can be assigned a value of any type. (This is the terminal object in the category of Zeolite types.)
all: Value type that can be assigned to all other types. (This is the initial object in the category of Zeolite types.)

Builtin Constants

empty (optional all): A missing optional value.
false (Bool): Obvious.
self (#self): The value being operated on in @value functions.
true (Bool): Obvious.

Builtin Functions

identify: Returns an Identifier<#x> for the value.
present: Check optional for empty.
reduce<#x, #y>(value): See Runtime Type Reduction.
require: Convert optional to non-optional.
strong: Convert weak to optional.
typename<#x>(): Formats the real type of #x as a Formatted value.

Procedural Operators

Operators	Semantics	Example	Input Types	Result Type	Notes
`+`,`-`,`*`,`/`	arithmetic	`x + y`	`Int`,`Float`	original type
`%`	arithmetic	`x % y`	`Int`	`Int`
`-`	arithmetic	`x - y`	`Char`	`Int`
`^`,`\|`,`&`,`<<`,`>>`,`~`	bit operations	`x ^ y`	`Int`	`Int`
`+`	concatenation	`x + y`	`String`	`String`
`^`,`!`,`\|\|`,`&&`	logical	`x && y`	`Bool`	`Bool`
`<`,`>`,`<=`,`==`,`>=`,`!=`	comparison	`x < y`	built-in unboxed, `String`	`Bool`	not available for `Pointer`
`.`	function call	`x.foo()`	value	function return type(s)
`.`	function call	`T.foo()`	type instance	function return type(s)
`:`	function call	`T:foo()`	category	function return type(s)
`&.`	conditional function call	`x&.foo()`	`optional` value	function return type(s) converted to `optional`	skips evaluation of args if call is skipped
`?`	type conversion	`x?T`	left: value right: type instance	right type with optionality of left	can also be used with `optional` values
`<-`	assignment	`x <- y`	left: variable right: expression	right type
`<-\|`	conditional assignment	`x <-\| y`	left: `optional` variable right: non-`weak` expression	left type with optionality of right	skips evaluation of right if left is `present`
`<\|\|`	fallback value	`x <\|\| y`	left: `optional` expression right: non-`weak` expression	union of left and right types with optionality of right	skips evaluation of right if left is `present`

Layout and Dependencies

Using Public Source Files

You can create public .0rp source files to declare concrete categories and interfaces that are available for use in other sources. This is the only way to share code between different source files. .0rp cannot contain defines for concrete categories.

During compilation, all .0rp files in the project directory are loaded up front. This is then used as the set of public symbols available when each .0rx is separately compiled.

Standard Library

The standard library currently temporary and lacks a lot of functionality. See the public .0rp sources in lib. Documentation will eventually follow.

Modules

You can depend on another module using -i lib/util for a public dependency and -I lib/util for a private dependency when calling zeolite. (A private dependency is not visible to modules that depend on your module.)

Dependency paths are first checked relative to the module depending on them. If the dependency is not found there, the compiler then checks the global location specified by zeolite --get-path.

Public .0rp source files are loaded from all dependencies during compilation, and so their symbols are available to all source files in the module. There is currently no language syntax for explicitly importing or including modules or other symbols.

If you are interested in backing a concrete category with C++, you will need to write a custom .zeolite-module file. Better documentation will eventually follow, but for now:

Create a .0rp with declarations of all of the concrete categories you intend to define in C++ code.
Run zeolite in --templates mode to generate .cpp templates for all concrete categories that lack a definition in your module.
Run zeolite in -c mode to get a basic .zeolite-module. After this, always use recompile mode (-r) to use your .zeolite-module.
Take a look at .zeolite-module in lib/file to get an idea of how to tell the compiler where your category definitions are.
Add your code to the generated .cpp files. lib/file is also a reasonable example for this.
If you need to depend on external libraries, fill in the include_paths and link_flags sections of .zeolite-module.

IMPORTANT: @value functions for immutable categories will be marked as const in C++ extensions. immutable also requires that @value members have immutable types, but there is no reasonable way to enforce this in C++. You will need to separately ensure that the implementation only stores other immutable types, just for consistency with categories implemented in Zeolite.

Unit Testing

Unit testing is a built-in capability of Zeolite. Unit tests use .0rt source files, which are like .0rx source files with testcase metadata. The test files go in the same directory as the rest of your source files. (Elsewhere in this project these tests are referred to as "integration tests" because this testing mode is used to ensure that the zeolite compiler operates properly end-to-end.)

Writing Tests

IMPORTANT: Prior to compiler version 0.10.0.0, the testcase syntax was slightly different, and unittest was not available.

// myprogram/tests.0rt

// Each testcase starts with a header specifying a name for the group of tests.
// This provides common setup code for a group of unit tests.

testcase "passing tests" {
  // All unittest are expected to execute without any issues.
  success
}

// Everything after the testcase (up until the next testcase) is like a .0rx.

// At least one unittest must be defined when success is expected. Each unittest
// must have a distinct name within the testcase. Each unittest is run in a
// separate process, making it safe to alter global state.

unittest myTest1 {
  // The test content goes here. It has access to anything within the testcase
  // besides other unittest.
}

unittest myTest2 {
  \ empty
}


// A new testcase header indicates the end of the previous test.

testcase "missing function" {
  // The test is expected to have a compilation error. Note that this cannot be
  // used to check for parser failures!
  //
  // Any testcase can specify require and exclude regex patterns for checking
  // test output. Each pattern can optionally be qualified with one of compiler,
  // stderr, or stdout, to specify the source of the output.
  error
  require compiler "run"  // The compiler error should include "run".
  exclude compiler "foo"  // The compiler error should not include "foo".
}

// You can include unittest when an error is expected; however, they will not be
// run even if compilation succeeds.

define MyType {
  // Error! MyType does not have a definition for run.
}

concrete MyType {
  @type run () -> ()
}


testcase "intentional failure" {
  // The test is expected to fail.
  failure
  require stderr "message"  // stderr should include "message".
}

// Exactly one unittest must be defined when a failure is expected.

unittest myTest {
  // Use the fail built-in to cause a test failure.
  fail("message")
}


testcase "compilation tests" {
  // Use compiles to check only Zeolite compilation, with no C++ compilation or
  // execution of tests.
  compiles
}

unittest myTest {
  // unittest is optional in this mode, but can still be used if the tests does
  // not require any new types.
}

Unit tests have access to all public and $ModuleOnly$ symbols in the module. You can run all tests for module myprogram using zeolite -t myprogram.

Specific things to keep in mind with testcase:

All individual unittest have a default timeout of 30 seconds. This is to prevent automated test runners from hanging indefinitely if there is a deadlock or an infinite loop. This can be changed with timeout t, where t is specified in seconds. Specifing timeout 0 disables the time limit.
To simplify parsing, there are a few limitations with how you order the fields in a testcase:
- The expected outcome (success, failure, error, compiles) must be at the top. (Prior to compiler version 0.24.0.0, crash was used instead of failure.)
- All require and exclude patterns must be grouped together. (Put another way, if a field other than require or exclude follows one of those two, the parser will move on.)

Code Coverage

As of compiler version 0.16.0.0, you can get a log of all lines of Zeolite code (from .0rx or .0rt sources) with the --log-traces [filename] option when running tests with zeolite -t.

If [filename] is not an absolute path, it will be created relative to the path specified with -p if used. The file will be overwritten, and will contain all traces from a single call to zeolite -t.
The current format is .csv with the following columns (includes a header):
- "microseconds": The call time from a monotonic microseconds timer, with an unspecified starting point.
- "pid": A unique ID for the current process. This is not the real process ID from the system, since that will often not be unique.
- "function": The name of the function where the line was executed.
- "context": The source-code context (e.g., file, line) of the call. The context has the same format as stack traces for crashes.
If a procedure uses the $NoTrace$ pragma, there will be no trace information in the log for that procedure. This is because the logging uses the same tooling that is used for stack traces. Similarly, if a specific unittest uses the $DisableCoverage$ pragma, no coverage will be recorded as a result of that unittest.
Nothing will be logged for testcase that use compiles or error, since those modes do not actually execute any compiled code.
Keep in mind that the simple act of processing text for logging can obscure race conditions in a program; therefore, --log-traces should be skipped when troubleshooting race conditions.
Check the size of the log file before attempting to open it in a desktop application. In many cases, it will be too large to display.

As of compiler version 0.20.0.0, zeolite -r will cache information about the possible .0rx lines that can show up in --log-traces mode.

You can access this information using zeolite --show-traces [module path]. This can then be compared to the "context" field in the output .csv to determine if any code was missed by the executed tests.
Alternatively, you can have zeolite compute the missed lines using zeolite --missed-lines [filename] [module path], where [filename] is the file written by --log-traces. If a single zeolite -t command executed tests for multiple modules, you can pass all of those modules to a single zeolite --missed-lines call.

For example:
```
# Run the tests.
zeolite -t --log-traces traces.csv your/module1 your/module2

# Output the line numbers missed by the tests.
zeolite --missed-lines traces.csv your/module1 your/module2
```
Note that --missed-lines does not account for which module's test was responsible for covering a line during the tests. For example, if you run zeolite -t --log-traces traces.csv your/module1 your/module2 and your/module2 depends on your/module1, tests from your/module2 that execute your/module1 code will contribute to your/module1's coverage.

Compiler Pragmas and Macros

(As of compiler version 0.5.0.0.)

Pragmas allow compiler-specific directives within source files that do not otherwise need to be a part of the language syntax. Macros have the same format, and are used to insert code after parsing but before compilation.

The syntax for both is $SomePragma$ (no options) or $AnotherPragma[OPTIONS]$ (uses pragma-specific options). The syntax for OPTIONS depends on the pragma being used. Pragmas are specific to the context they are used in.

Source File Pragmas

These must be at the top of the source file, before declaring or defining categories or testcases.

$ModuleOnly$ . This can only be used in .0rp files. It takes an otherwise-public source file and limits visibility to the module. (This is similar to package-private in Java.)
$TestsOnly$ . This can be used in .0rp and .0rx files. When used, the file is only visible to other sources that use it, as well as .0rt sources. .0rp sources still remain public unless $ModuleOnly$ is used. The transitive effect of $TestsOnly$ is preventing the use of particular categories in output binaries.

Procedure Pragmas

These must occur at the very top of a function definition.

$NoTrace$ . (As of compiler version 0.6.0.0.) Disables stack-tracing within this procedure. This is useful for recursive functions, so that trace information does not take up stack space. This does not affect tracing for functions that are called from within the procedure.
$TraceCreation$ . (As of compiler version 0.6.0.0.) Includes a trace of the value's creation when the given @value function is called. If multiple functions in a call stack use $TraceCreation$ , only the trace from the bottom-most function will be included.

$TraceCreation$ is useful when the context that the value was created in is relevant when debugging crashes. The added execution cost for the function is trivial; however, it increases the memory size of the value by a few bytes per call currently on the stack at the time it gets created.

`define` Pragmas

These must be at the top of a category define immediately following {.

$FlatCleanup[memberName]$. (As of compiler version 0.21.0.0.) Clear the @value member memberName before actually cleaning up the @value itself to avoid recursive cleanup. Use this when recursive cleanup might otherwise result in a stack overflow, e.g., with linked lists.

Only one member can be specified in this pragma so that the implementation does not need to use a dynamically-sized cleanup queue to handle branching.
$ReadOnly[var1, var2, ...]$. (As of compiler version 0.16.0.0.) See Local Variable Rules. Only applies to @category and @value members.
$Hidden[var1, var2, ...]$. (As of compiler version 0.16.0.0.) See Local Variable Rules. Only applies to @category and @value members.
$ReadOnlyExcept[var1, var2, ...]$. (As of compiler version 0.22.1.0.) Marks all @category and @value members as read-only except those listed.
- If multiple ReadOnlyExcept are used, they are unioned rather than intersected. For example, $ReadOnlyExcept[foo]$ and $ReadOnlyExcept[bar]$ together make $ReadOnlyExcept[foo, bar]$ and not $ReadOnlyExcept[]$ .
- If a variable is listed in both ReadOnly and ReadOnlyExcept, the variable is marked as read-only.

`unittest` Pragmas

These must be at the top of a unittest immediately following {.

$DisableCoverage$ . (As of compiler version 0.20.0.0.) Disables all collection of code coverage when --log-traces is used. This is useful if a particular unit test causes a massive number of lines to be executed.

Local Variable Rules

These pragmas alter how variables are dealt with locally:

$ReadOnly[var1, var2, ...]$. (As of compiler version 0.13.0.0.) Marks var1, var2, etc. as read-only for the remainder of the statements in this context. Note that this only prevents assignment to the variable; it does not prevent making calls to functions that change the state of the underlying value. It also does not prevent calls to other functions modifying the variable.
```
scoped {
  Int i <- 0
} in while (i < 100) {
  // i can still be overwritten here
  $ReadOnly[i]$
  // i can not be overwritten below here within the while block
} update {
  // this is fine because it's in a different scope
  i <- i+1
}
```
This can be used for any variable name visible in the current scope, including @value and @category members and argument and return variables.
$Hidden[var1, var2, ...]$. (As of compiler version 0.13.0.0.) This works the same way as ReadOnly except that it also makes the variables inaccessible for reading. Note that this does not allow you to reuse a variable name; the variable name remains reserved.

As of compiler version 0.16.0.0 both $ReadOnly[...]$ and $Hidden[...]$ can be used at the top of a define for a concrete category to protect member variables.

define Type {
  $Hidden[foo]$
  $ReadOnly[bar]$

  @category Int foo <- 1
  // foo can still be read here for the purposes of initialization.
  @category Int bar <- foo
}

Zeolite uses pragmas instead of something like final in Java for a few reasons:

In practice, a large percentage of variables to be treated as read-only require some sort of iterative or conditional setup. Marking the variable as final at declaration time would require creating a helper to initialize it.
```
Int nextPow2 <- 1
while (nextPow2 < n) {
  nextPow2 <- 2*nextPow2
}
$ReadOnly[nextPow2]$
```
A frequent source of errors in any language is multiple variables of the same type in the same scope. Marking some of them as read-only (or as hidden) within a certain context helps prevent errors caused by inadvertently using the wrong variable.
If your code works correctly with variables marked as read-only or as hidden, then it will also work correctly without such markings; the markings are only there to cause compile-time errors. This means that they add limited value to interpreting the code, and can therefore be kept separate from the respective variable's definition without loss of clarity.

Expression Macros

These can be used in place of language expressions.

$SourceContext$ . (As of compiler version 0.7.1.0.) Inserts a String-literal with information about the macro's location within the source file. Note that if this is used within an expression macro in .zeolite-module (see ExprLookup below), the context will be within the .zeolite-module file itself. (Remember that macro substitution is not a preprocessor stage, unlike the C preprocessor.)
$CallTrace$ . (As of compiler version 0.24.0.0.) Inserts an optional Order<Formatted> containing the current call trace. This is only available in .0rt test files and in .0rx with $TestsOnly$ . This cannot be used outside of non-test code, just so that logic can't depend on where the call originated from.
$ExprLookup[MACRO_NAME]$. (As of compiler version 0.6.0.0.) This directly substitutes in a language expression, as if it was parsed from that exact code location. MACRO_NAME is the key used to look up the expression. Symbols will be resolved in the context that the substutition happens in.
- MODULE_PATH is always defined. It is a String-literal containing the absolute path to the module owning the source file. This can be useful for locating data directories within your module independently of $PWD.
- Custom macros can be included in the .zeolite-module for your module. This can be useful if your module requires different parameters from one system to another.
```
// my-module/.zeolite-module

// (Standard part of .zeolite-module.)
path: "."

// Define your macros here.
expression_map: [
  expression_macro {
    name: USE_DATA_VERSION    // Access using $ExprLookup[USE_DATA_VERSION]$.
    expression: "2020-05-12"  // Substituted in as a Zeolite expression.
  }
  expression_macro {
    name: RECURSION_LIMIT
    expression: 100000
  }
  expression_macro {
    name: SHOW_LIMIT
    // All Zeolite expressions are allowed.
    expression: "limit: " + $ExprLookup[RECURSION_LIMIT]$.formatted()
  }
]

// (Standard part of .zeolite-module.)
mode: incremental { }
```
  The name: must only contain uppercase letters, numbers, and _, and the expression: must parse as a valid Zeolite expression. This is similar to C++ macros, except that the substitution must be independently parsable as a valid expression, and it can only be used where expressions are otherwise allowed.

Known Language Limitations

Reference Counting

Zeolite currently uses reference counting rather than a garbage-collection system that might otherwise search for unreferenced objects in the background. While this simplifies the implementation, it is possible to have a reference cycle that prevents cleanup of the involved objects, thereby causing a memory leak.

This can be mitigated by using weak references in categories where a cycle is probable or guaranteed. For example, LinkedNode in lib/container is a doubly-linked list, which would create a reference cycle if both forward and reverse references were non-weak.

Name		Name	Last commit message	Last commit date
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.github/workflows		.github/workflows
base		base
bin		bin
capture-thread @ 63e53be		capture-thread @ 63e53be
docs		docs
editors		editors
example		example
lib		lib
scripts		scripts
src		src
tests		tests
.gitignore		.gitignore
.gitmodules		.gitmodules
ChangeLog.md		ChangeLog.md
LICENSE		LICENSE
README.md		README.md
Setup.hs		Setup.hs
zeolite-lang.cabal		zeolite-lang.cabal

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