check

Namespace: FsToolkit.ErrorHandling

The intent of check is to allow an Ok result value to be validated.

check takes a validation function of the form 'ok -> Task<Result<unit, 'error>> and a result of the form Task<Result<'ok, 'error>>.

If the task-wrapped result is Ok x then the validation function is applied, and if the validation function returns an error, this new task-wrapped error is returned. Otherwise, the original task-wrapped Ok x result is returned. If the original task-wrapped result is an Error, the original task-wrapped result is returned.

Function Signature

('ok -> Task<Result<unit,'error>>) -> Task<Result<'ok,'error>> -> Task<Result<'ok,'error>>

Examples

Given the following function that returns true for the id 123

checkEnabled : int -> Task<bool>

Example 1

TaskResult.ok (
    {|
        PolicyId = 123
        AccessPolicyName = "UserCanAccessResource"
    |}

)
|> TaskResult.check (fun policy ->
    taskResult {
        let! isEnabled = checkEnabled policy.PolicyId

        return
            if not isEnabled then
                Error(
                    $"The policy {policy.AccessPolicyName} cannot be used because its disabled."
                )
            else
                Ok()

    }
)
// TaskResult.Ok {| AccessPolicyName = "UserCanAccessResource"; IsEnabled = true; |}

Example 2

TaskResult.ok (
    {|
        PolicyId = 456
        AccessPolicyName = "UserCanAccessResource"
    |}

)
|> TaskResult.check (fun policy ->
    taskResult {
        let! isEnabled = checkEnabled policy.PolicyId

        return
            if not isEnabled then
                Error(
                    $"The policy {policy.AccessPolicyName} cannot be used because its disabled."
                )
            else
                Ok()

    }
)

// TaskResult.Error "The policy UserCanAccessResource cannot be used because its disabled."