Ferrum

The F# library for working with dynamic Error types.

The first library goal is providing lightweight general dynamic error type IError that can be used in Result error case (Result<'a, IError>) instead string or exn (Exception). The string error type does not compose well; exn is composable, but it is heavier and looks strange when not thrown. IError type is something in between.

The second library goal is providing contextual enrichment for errors.

Examples

IError usages

Ferrum provides 3 fundamental error types: message, context, wrap.

• message is error that is created first and causes the all errors chain.
• context is error that add context to other error.
• wrap is error that wrap any 'e type to IError. It useful if impossible or too expensive to implement IError interface for 'e. 'e can be exception, but recommended use Error.ofException for converting exn -> IError.

let messageError: IError = Error.message "This is some error"
printfn $" > {messageError.FormatChain()}"
// > This is some error

let contextualError: IError = Error.context "Top error" (Error.message "Root error")
printfn $" > {contextualError.FormatChain()}"
// > Top error: Root error

type SomeError = SomeError
let wrappedError: IError = Error.wrap SomeError
printfn $" > {wrappedError.FormatChain()}"
// > SomeError

All operation are duplicated on Result module:

let messageResult: Result<unit, IError> = Result.message "This is some error"
printfn $" > {Utils.formatResultChain messageResult}"
// > Error (This is some error)

let contextualResult: Result<unit, IError> = Result.context "Top error" (Result.message "Root error")
printfn $" > {Utils.formatResultChain contextualResult}"
// > Error (Top error: Root error)

type SomeError = SomeError
let wrappedResult: Result<unit, IError> = Result.wrap (Error SomeError)
printfn $" > {Utils.formatResultChain wrappedResult}"
// > Error (SomeError)

Complex usage

[<RequireQualifiedAccess>]
type IOError =
    | FileNotFound

let readFile (_fileName: string) : Result<string, IOError> =
    Error IOError.FileNotFound

let makeGreeting () : Result<string, IError> =
    let readNameResult = readFile "name.txt" |> Result.wrap |> Result.context "Name is unknown"
    match readNameResult with
    | Error err -> Error err
    | Ok name -> Ok $"Hello, {name}"

let greet () : Result<unit, IError> =
    let makeGreetingResult = makeGreeting () |> Result.context "Greeting is not build"
    match makeGreetingResult with
    | Error err -> Error err
    | Ok greeting ->
        printfn $"{greeting}"
        Ok ()

let res: Result<unit, IError> = greet ()
printfn $" > {Utils.formatResultChain res}"
// > Error (Greeting is not build: Name is unknown: FileNotFound)

Custom IError implementation

type SimpleError =
    | SimpleCase
    with
        interface IError with
            member this.Reason =
                match this with
                | SimpleCase -> "Some simple error case"
            member this.Source =
                ValueNone

type ComplexError =
    | Source of SimpleError
    | SomeError
    with
        interface IError with
            member this.Reason =
                match this with
                | Source _ -> "Error caused by simple error source"
                | SomeError -> "Some complex error case"
            member this.Source =
                match this with
                | Source simpleError -> ValueSome simpleError
                | SomeError -> ValueNone

let simpleError = SimpleError.SimpleCase
printfn $" > {simpleError.Format(ErrorFormatters.ChainErrorFormatter.Instance)}"
// > Some simple error case

let complexErrorSomeError = ComplexError.SomeError
printfn $" > {complexErrorSomeError.Format(ErrorFormatters.ChainErrorFormatter.Instance)}"
// > Some complex error case

let complexErrorWithSource = ComplexError.Source SimpleError.SimpleCase
printfn $" > {complexErrorWithSource.Format(ErrorFormatters.ChainErrorFormatter.Instance)}"
// > Error caused by simple error source: Some simple error case

Why not Exceptions?

Exceptions have all the properties that IErrors have. Literally Reason <=> Message, Source <=> InnerException, StackTrace <=> StackTrace. The only difference is that IError is a bit more focused on using in Result and is possibly more lightweight. It is easy to implement a similar utilities for exn. It all comes down to stylistic preferences.

Additional Result and Error functions

At the moment, Ferrum has no goals to cover the usability of error handling beyond the universal error type. FsToolkit.ErrorHandling and other libraries do a great job with this.