> For the complete documentation index, see [llms.txt](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/fstoolkit.errorhandling.icedtasks/index/ce.md).

# Computation Expression

### CancellableTaskResult Computation Expression

Namespace: `FsToolkit.ErrorHandling`

### CancellableTaskResult Examples

#### Example 1

The initial [motivating example](/fstoolkit-errorhandling/readme.md) is perhaps more realistic if some functions are asynchronous. Given the following functions:

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

A login flow can be implemented as below using the `cancellableTaskResult` CE and a few [helpers](/fstoolkit-errorhandling/fstoolkit.errorhandling.icedtasks/index/others.md):

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

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

### BackgroundCancellableTaskResults

You also have an option to use the 'backgroundCancellableTaskResult' computation expression. It will still use the CancellableTaskResult type, but it will escape to a background thread where necessary.