Computation Expression

AsyncResult Computation Expression

Namespace: FsToolkit.ErrorHandling

Examples

Example 1

The initial motivating example is perhaps more realistic if some functions are asynchronous. Given the following functions:

tryGetUser : string -> Async<User option>
isPwdValid : string -> User -> bool
authorize : User -> Async<Result<unit, AuthError>>
createAuthToken : User -> Result<AuthToken, TokenError>

A login flow can be implemented as below using the asyncResult CE and a few helpers:

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

let login (username: string) (password: string) : Async<Result<AuthToken, LoginError>> =
  asyncResult {
    let! user = username |> tryGetUser |> AsyncResult.requireSome InvalidUser
    do! user |> isPwdValid password |> Result.requireTrue InvalidPwd
    do! user |> authorize |> AsyncResult.mapError Unauthorized
    return! user |> createAuthToken |> Result.mapError TokenErr
  }