Computation Expression

CancellableTaskResult Computation Expression

Namespace: FsToolkit.ErrorHandling

CancellableTaskResult Examples

Example 1

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

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:

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


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.

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