FsToolkit.ErrorHandling is a utility library to work with the
Resulttype in F#, and allows you to do clear, simple and powerful error handling.
The library provides utility functions like
sequenceas well as computation expressions and infix operators to work with
Result<'a option, 'b>,
Async<Result<'a option, 'b>>, and
Result<'a, 'b list>.
With adding a reference to
FsToolkit.ErrorHandling.TaskResultyou will also have access to computation expressions and infix operators related to
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
sequenceand 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>> =
// 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