feat: Add more slice methods to the stdlib (#5424)

# Description

## Problem\*

## Summary\*

Adds `filter` and `join` for slices to the stdlib along with the new
`Append` trait to abstract over joining types.

## Additional Context

These are methods I found would be useful while working on
metaprogramming

## Documentation\*

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/parser/parser.rs

@@ -1117,16 +1117,11 @@ where
 }
 
 fn quote() -> impl NoirParser<ExpressionKind> {
-    token_kind(TokenKind::Quote).validate(|token, span, emit| {
-        let tokens = match token {
+    token_kind(TokenKind::Quote).map(|token| {
+        ExpressionKind::Quote(match token {
             Token::Quote(tokens) => tokens,
             _ => unreachable!("token_kind(Quote) should guarantee parsing only a quote token"),
-        };
-        emit(ParserError::with_reason(
-            ParserErrorReason::ExperimentalFeature("quoted expressions"),
-            span,
-        ));
-        ExpressionKind::Quote(tokens)
+        })
     })
 }
 

docs/docs/noir/concepts/data_types/slices.mdx

@@ -197,7 +197,7 @@ fn main() {
 Applies a function to each element of the slice, returning a new slice containing the mapped elements.
 
 ```rust
-fn map<U>(self, f: fn(T) -> U) -> [U]
+fn map<U, Env>(self, f: fn[Env](T) -> U) -> [U]
 ```
 
 example
@@ -213,7 +213,7 @@ Applies a function to each element of the slice, returning the final accumulated
 parameter is the initial value.
 
 ```rust
-fn fold<U>(self, mut accumulator: U, f: fn(U, T) -> U) -> U
+fn fold<U, Env>(self, mut accumulator: U, f: fn[Env](U, T) -> U) -> U
 ```
 
 This is a left fold, so the given function will be applied to the accumulator and first element of
@@ -247,7 +247,7 @@ fn main() {
 Same as fold, but uses the first element as the starting element.
 
 ```rust
-fn reduce(self, f: fn(T, T) -> T) -> T
+fn reduce<Env>(self, f: fn[Env](T, T) -> T) -> T
 ```
 
 example:
@@ -260,12 +260,72 @@ fn main() {
 }
 ```
 
+### filter
+
+Returns a new slice containing only elements for which the given predicate returns true.
+
+```rust
+fn filter<Env>(self, f: fn[Env](T) -> bool) -> Self
+```
+
+example:
+
+```rust
+fn main() {
+    let slice = &[1, 2, 3, 4, 5];
+    let odds = slice.filter(|x| x % 2 == 1);
+    assert_eq(odds, &[1, 3, 5]);
+}
+```
+
+### join
+
+Flatten each element in the slice into one value, separated by `separator`.
+
+Note that although slices implement `Append`, `join` cannot be used on slice
+elements since nested slices are prohibited.
+
+```rust
+fn join(self, separator: T) -> T where T: Append
+```
+
+example:
+
+```rust
+struct Accumulator {
+    total: Field,
+}
+
+// "Append" two accumulators by adding them
+impl Append for Accumulator {
+    fn empty() -> Self {
+        Self { total: 0 }
+    }
+
+    fn append(self, other: Self) -> Self {
+        Self { total: self.total + other.total }
+    }
+}
+
+fn main() {
+    let slice = &[1, 2, 3, 4, 5].map(|total| Accumulator { total });
+
+    let result = slice.join(Accumulator::empty());
+    assert_eq(result, Accumulator { total: 15 });
+
+    // We can use a non-empty separator to insert additional elements to sum:
+    let separator = Accumulator { total: 10 };
+    let result = slice.join(separator);
+    assert_eq(result, Accumulator { total: 55 });
+}
+```
+
 ### all
 
 Returns true if all the elements satisfy the given predicate
 
 ```rust
-fn all(self, predicate: fn(T) -> bool) -> bool
+fn all<Env>(self, predicate: fn[Env](T) -> bool) -> bool
 ```
 
 example:
@@ -283,7 +343,7 @@ fn main() {
 Returns true if any of the elements satisfy the given predicate
 
 ```rust
-fn any(self, predicate: fn(T) -> bool) -> bool
+fn any<Env>(self, predicate: fn[Env](T) -> bool) -> bool
 ```
 
 example:

docs/docs/noir/standard_library/traits.md

@@ -51,6 +51,7 @@ For primitive integer types, the return value of `default` is `0`. Container
 types such as arrays are filled with default values of their element type,
 except slices whose length is unknown and thus defaulted to zero.
 
+---
 
 ## `std::convert`
 
@@ -85,6 +86,7 @@ For this reason, implementing `From` on a type will automatically generate a mat
 
 `Into` is most useful when passing function arguments where the types don't quite match up with what the function expects. In this case, the compiler has enough type information to perform the necessary conversion by just appending `.into()` onto the arguments in question.
 
+---
 
 ## `std::cmp`
 
@@ -178,6 +180,8 @@ impl<A, B, C, D, E> Ord for (A, B, C, D, E)
     where A: Ord, B: Ord, C: Ord, D: Ord, E: Ord { .. }
 ```
 
+---
+
 ## `std::ops`
 
 ### `std::ops::Add`, `std::ops::Sub`, `std::ops::Mul`, and `std::ops::Div`
@@ -301,3 +305,29 @@ impl Shl for u16 { fn shl(self, other: u16) -> u16 { self << other } }
 impl Shl for u32 { fn shl(self, other: u32) -> u32 { self << other } }
 impl Shl for u64 { fn shl(self, other: u64) -> u64 { self << other } }
 ```
+
+---
+
+## `std::append`
+
+### `std::append::Append`
+
+`Append` can abstract over types that can be appended to - usually container types:
+
+#include_code append-trait noir_stdlib/src/append.nr rust
+
+`Append` requires two methods:
+
+- `empty`: Constructs an empty value of `Self`.
+- `append`: Append two values together, returning the result.
+
+Additionally, it is expected that for any implementation:
+
+- `T::empty().append(x) == x`
+- `x.append(T::empty()) == x`
+
+Implementations:
+```rust
+impl<T> Append for [T]
+impl Append for Quoted
+```

noir_stdlib/src/append.nr

@@ -0,0 +1,35 @@
+// Appends two values together, returning the result.
+//
+// An alternate name for this trait is `Monoid` if that is familiar.
+// If not, it can be ignored.
+//
+// It is expected that for any implementation:
+// - `T::empty().append(x) == x`
+// - `x.append(T::empty()) == x`
+// docs:start:append-trait
+trait Append {
+    fn empty() -> Self;
+    fn append(self, other: Self) -> Self;
+}
+// docs:end:append-trait
+
+impl<T> Append for [T] {
+    fn empty() -> Self {
+        &[]
+    }
+
+    fn append(self, other: Self) -> Self {
+        // Slices have an existing append function which this will resolve to.
+        self.append(other)
+    }
+}
+
+impl Append for Quoted {
+    fn empty() -> Self {
+        quote {}
+    }
+
+    fn append(self, other: Self) -> Self {
+        quote { $self $other }
+    }
+}

noir_stdlib/src/lib.nr

@@ -27,6 +27,7 @@ mod uint128;
 mod bigint;
 mod runtime;
 mod meta;
+mod append;
 
 // Oracle calls are required to be wrapped in an unconstrained function
 // Thus, the only argument to the `println` oracle is expected to always be an ident

noir_stdlib/src/slice.nr

@@ -1,3 +1,5 @@
+use crate::append::Append;
+
 impl<T> [T] {
     #[builtin(array_len)]
     pub fn len(self) -> u32 {}
@@ -85,6 +87,33 @@ impl<T> [T] {
         accumulator
     }
 
+    // Returns a new slice containing only elements for which the given predicate
+    // returns true.
+    pub fn filter<Env>(self, predicate: fn[Env](T) -> bool) -> Self {
+        let mut ret = &[];
+        for elem in self {
+            if predicate(elem) {
+                ret = ret.push_back(elem);
+            }
+        }
+        ret
+    }
+
+    // Flatten each element in the slice into one value, separated by `separator`.
+    pub fn join(self, separator: T) -> T where T: Append {
+        let mut ret = T::empty();
+
+        if self.len() != 0 {
+            ret = self[0];
+
+            for i in 1..self.len() {
+                ret = ret.append(separator).append(self[i]);
+            }
+        }
+
+        ret
+    }
+
     // Returns true if all elements in the slice satisfy the predicate
     pub fn all<Env>(self, predicate: fn[Env](T) -> bool) -> bool {
         let mut ret = true;

test_programs/compile_success_empty/slice_join/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "slice_join"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.31.0"
+
+[dependencies]

test_programs/compile_success_empty/slice_join/src/main.nr

@@ -0,0 +1,16 @@
+use std::append::Append;
+
+fn main() {
+    let slice = &[1, 2, 3, 4, 5];
+
+    let odds = slice.filter(|x| x % 2 == 1);
+    assert_eq(odds, &[1, 3, 5]);
+
+    let odds_and_evens = append_three(odds, &[100], &[2, 4]);
+    assert_eq(odds_and_evens, &[1, 3, 5, 100, 2, 4]);
+}
+
+fn append_three<T>(one: T, two: T, three: T) -> T where T: Append {
+    // The `T::empty()`s here should do nothing
+    T::empty().append(one).append(two).append(three).append(T::empty())
+}