FsToolkit.ErrorHandling

FsToolkit.ErrorHandling is a utility library to work with the Result type in F#, and allows you to do clear, simple and powerful error handling.

The library provides utility functions like map, bind, apply, traverse, sequence as well as computation expressions and infix operators to work with Result<'a, 'b>, Result<'a option, 'b>, Async<Result<'a, 'b>>, Async<Result<'a option, 'b>>, and Result<'a, 'b list>.

It was inspired by Chessie and Cvdm.ErrorHandling (the latter has now been merged into FsToolkit.ErrorHandling).

FsToolkit.ErrorHandling targets .NET Standard 2.0 and .NET Framework 4.6.1 and supports Fable.

Documentation

The documentation is available here.

Further material

• The main resource as to learning this style of programming Railway Oriented Programming
• However Result isn't a panacea, see what pitfalls and where you shouldn't use Result. In defense of Exceptions: Throw (away) your Result

Builds

GitHub Actions

NuGet

Package name	Release	Prelease
FsToolkit.ErrorHandling
FsToolkit.ErrorHandling.TaskResult
FsToolkit.ErrorHandling.JobResult
FsToolkit.ErrorHandling.AsyncSeq
FsToolkit.ErrorHandling.IcedTasks

Developing locally

Devcontainer

This repository has a devcontainer setup for VSCode. For more infomation see:

• VSCode

Local Setup

• .NET Core SDK
  • v6.x
  • v7.x

Optional

To test fable builds locally you will need:

• Node
  • v18.0.0 or Higher
  • Not required but recommend that you use NVM to easily manage multiple versions of Node
• Python
  • v3.10.0 or higher
  • Required for Fable-Python

Compiling

> build.cmd <optional buildtarget> // on windows
$ ./build.sh  <optional buildtarget>// on unix

Without specifying a build target, the default target is DotnetPack, which will run tests for all projects on dotnet and then pack the projects into nuget packages. For additional notable targets see below.

Build Targets

• Clean - Will clean all projects bin and obj folders
• DotnetTest - Will run tests for dotnet projects
• NpmTest - Will run tests for fable-javascript projects
• PythonTest - Will run tests for fable-python projects
• RunTests - Will run tests for dotnet, fable-javascript and fable-python projects
• FormatCode - Will run fantomas to format the codebase

This is not an exhausting list. Additional targets can be found in the ./build/build.fs file.

A motivating example

This example of composing a login flow shows one example of how this library can aid in clear, simple, and powerful error handling, using just a computation expression and a few helper functions. (The library has many more helper functions and computation expressions as well as infix operators; see the documentation for details.)

// Given the following functions:
//   tryGetUser: string -> Async<User option>
//   isPwdValid: string -> User -> bool
//   authorize: User -> Async<Result<unit, AuthError>>
//   createAuthToken: User -> Result<AuthToken, TokenError>

type LoginError = InvalidUser | InvalidPwd | Unauthorized of AuthError | TokenErr of TokenError

let login (username: string) (password: string) : Async<Result<AuthToken, LoginError>> =
  asyncResult {
    // requireSome unwraps a Some value or gives the specified error if None
    let! user = username |> tryGetUser |> AsyncResult.requireSome InvalidUser

    // requireTrue gives the specified error if false
    do! user |> isPwdValid password |> Result.requireTrue InvalidPwd

    // Error value is wrapped/transformed (Unauthorized has signature AuthError -> LoginError)
    do! user |> authorize |> AsyncResult.mapError Unauthorized

    // Same as above, but synchronous, so we use the built-in mapError
    return! user |> createAuthToken |> Result.mapError TokenErr
  }

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