**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. DUBâ€‹

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 Wlaschinâ€™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>â€‹type LoginError = InvalidUser | InvalidPwd | Unauthorized of AuthError | TokenErr of TokenErrorâ€‹let login (username: string) (password: string) : Async<Result<AuthToken, LoginError>> =asyncResult {// requireSome unwraps a Some value or gives the specified error if Nonelet! user = username |> tryGetUser |> AsyncResult.requireSome InvalidUserâ€‹// requireTrue gives the specified error if falsedo! user |> isPwdValid password |> Result.requireTrue InvalidPwdâ€‹// Error value is wrapped/transformed (Unauthorized has signature AuthError -> LoginError)do! user |> authorize |> AsyncResult.mapError Unauthorizedâ€‹// Same as above, but synchronous, so we use the built-in mapErrorreturn! user |> createAuthToken |> Result.mapError TokenErr}

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