bind.md

CancellableTaskValidation.bind

Namespace: FsToolkit.ErrorHandling

Function Signature:

('input -> CancellableTaskValidation<'output, 'Error>)
  -> CancellableTaskValidation<'input, 'Error>
  -> CancellableTaskValidation<'output, 'Error>

Chains two CancellableTaskValidation computations using monadic (short-circuit) semantics. If the first computation returns an Error, the binder function is not called and the error is propagated immediately.

Note: Unlike apply or map2, bind does not accumulate multiple errors — it short-circuits on the first failure. For error accumulation, use the applicative operators or the cancellableTaskValidation CE with and!.

Examples

Note: Many use-cases requiring bind operations can also be solved using the cancellableTaskValidation computation expression.

Example 1

Sequentially validating a user exists before validating their permissions:

let validateUserWithPermissions (userId: UserId) : CancellableTaskValidation<UserWithRole, string> =
    fetchUser userId
    |> CancellableTaskValidation.bind (fun user -> fetchAndValidateRole user)

Example 2

Chaining validation steps in a pipeline:

let processPayment (request: PaymentRequest) : CancellableTaskValidation<Receipt, PaymentError> =
    validatePaymentRequest request
    |> CancellableTaskValidation.bind authorizePayment
    |> CancellableTaskValidation.bind chargeCard

Example 3

Using bind inside a cancellableTaskValidation CE with sequential steps:

let createUser (input: CreateUserInput) : CancellableTaskValidation<UserId, string> =
    cancellableTaskValidation {
        let! validatedEmail = validateEmail input.Email
        // bind is used implicitly here — short-circuits if validateEmail fails
        let! userId = createUserRecord validatedEmail input.Name
        return userId
    }