fold

Namespace: FsToolkit.ErrorHandling

Function Signature

('a -> 'b) -> ('c -> 'b) 
    -> Result<'a, 'c> -> 'b

Examples

Example 1

fold can be used to convert Result to another similar type, such as Choice:

let choice1 = Ok 42 |> Result.fold Choice1Of2 Choice2Of2
// Choice1Of2 42

let choice2 = Error "An error occurred" |> Result.fold Choice1Of2 Choice2Of2
// Choice2Of2 "An error occurred"

Example 2

In a typical web application, if there is any request validation error, we send HTTP 400 Bad Request as response and HTTP 200 OK for a successful operation.

Given the following function:

// string -> Result<int, string>
let tryParseInt str =
  match System.Int32.TryParse str with
  | true, x -> Ok x
  | false, _ -> 
    Error (sprintf "unable to parse '%s' to integer" str)

And the following fake HTTP response type:

type HttpResponse<'a, 'b> =
  | Ok of 'a
  | BadRequest of 'b

Then using Result.fold, we can do the following

// HttpRequest -> HttpResponse<int,string>
let handler httpRequest =
  // reading the input from the HTTP request
  let inputStr = httpRequest ... 
  inputStr |> tryParseInt |> Result.fold Ok BadRequest