# Computation Expression

### AsyncResult Computation Expression

Namespace: `FsToolkit.ErrorHandling`

### Examples

#### Example 1

The initial [motivating example](https://github.com/demystifyfp/FsToolkit.ErrorHandling/blob/master/README.md) is perhaps more realistic if some functions are asynchronous. Given the following functions:

```fsharp
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](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/fstoolkit.errorhandling/asyncresult/broken-reference):

```fsharp
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
  }
```