The Result<'T,'TFailure> type lets you write error-tolerant code that can be composed.


// The definition of Result in FSharp.Core
[<StructuralEquality; StructuralComparison>]
type Result<'T,'TError> =
    | Ok of ResultValue:'T
    | Error of ErrorValue:'TError


See the Result module for the built-in combinators for the Result. type.

Note that the result type is a struct discriminated union. Structural equality semantics apply here.

The Result type is typically used in monadic error-handling, which is often referred to as Railway-oriented Programming within the F# community. The following trivial example demonstrates this approach.

// Define a simple type which has fields that can be validated
type Request =
    { Name: string
      Email: string }

// Define some logic for what defines a valid name.
// Generates a Result which is an Ok if the name validates;
// otherwise, it generates a Result which is an Error.
let validateName req =
    match req.Name with
    | null -> Error "No name found."
    | "" -> Error "Name is empty."
    | "bananas" -> Error "Bananas is not a name."
    | _ -> Ok req

// Similarly, define some email validation logic.
let validateEmail req =
    match req.Email with
    | null -> Error "No email found."
    | "" -> Error "Email is empty."
    | s when s.EndsWith("") -> Error "No email from is allowed."
    | _ -> Ok req

let validateRequest reqResult =
    |> Result.bind validateName
    |> Result.bind validateEmail

let test() =
    // Now, create a Request and pattern match on the result.
    let req1 = { Name = "Phillip"; Email = "" }
    let res1 = validateRequest (Ok req1)
    match res1 with
    | Ok req -> printfn $"My request was valid! Name: {req.Name} Email {req.Email}"  
    | Error e -> printfn $"Error: {e}"
    // Prints: "My request was valid!  Name: Phillip Email:"

    let req2 = { Name = "Phillip"; Email = "" }
    let res2 = validateRequest (Ok req2)
    match res2 with
    | Ok req -> printfn $"My request was valid! Name: {req.Name} Email {req.Email}"  
    | Error e -> printfn $"Error: {e}"
    // Prints: "Error: No email from is allowed."


As you can see, it's quite easy to chain together various validation functions if you force them all to return a Result. This lets you break up functionality like this into small pieces which are as composable as you need them to be. This also has the added value of enforcing the use of pattern matching at the end of a round of validation, which in turns enforces a higher degree of program correctness.

See also