# README

**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>`

.With adding a reference to

`FsToolkit.ErrorHandling.TaskResult`

you will also have access to computation expressions and infix operators related to `Task<Result<'a, 'b>>`

.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.

This documentation assumes some familiarity with standard functions like

`map`

, `apply`

, `bind`

, `traverse`

and `sequence`

and the problem these functions solve. If this is new to you, check out Scott Wlaschin’s excellent tutorial on the subject.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 rest of 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

}

Last modified 2yr ago