bind.md

TaskValidation.bind

Namespace: FsToolkit.ErrorHandling

bind applies a function to the value inside a TaskValidation if it is Ok, returning the resulting TaskValidation. Like Validation.bind, this uses monadic (short-circuit) semantics — if the input is an Error, the binder function is not called and the error is returned immediately.

Function Signature

('okInput -> TaskValidation<'okOutput, 'error>) -> TaskValidation<'okInput, 'error> -> TaskValidation<'okOutput, 'error>

Examples

Take the following function for example:

// string -> TaskValidation<int, string>
let tryParseIntTask (s: string) =
    task {
        match System.Int32.TryParse(s) with
        | true, i -> return Ok i
        | false, _ -> return Error [ $"'%s{s}' is not a valid integer" ]
    }

Example 1

let result =
    TaskValidation.ok "42"
    |> TaskValidation.bind tryParseIntTask

// task { Ok 42 }

Example 2

let result =
    TaskValidation.ok "not a number"
    |> TaskValidation.bind tryParseIntTask

// task { Error ["'not a number' is not a valid integer"] }

Example 3

let result =
    TaskValidation.error "previous error"
    |> TaskValidation.bind tryParseIntTask

// task { Error ["previous error"] }