map2

Namespace: FsToolkit.ErrorHandling

Function Signature

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

Examples

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

Example 1

Let's assume that we have an add function that adds two numbers:

// int -> int -> int
let add a b = a + b

And an another function that converts a string to an integer:

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

With the help of Result.map2 function, we can now do the following:

let okResult =
  Result.map2 add (tryParseInt "40") (tryParseInt "2")
  // Ok 42

let errorResult =
  Result.map2 add (tryParseInt "40") (tryParseInt "foobar")
  // Error "unable to parse 'foobar' to integer"

Example 2

Let's assume that we have the following types:

Latitude

type Latitude = private Latitude of float with
  // float
  member this.Value = let (Latitude lat) = this in lat
  // float -> Result<Latitude, string>
  static member TryCreate (lat : float) =
    if lat > -90. && lat <= 90. then
      Ok (Latitude lat)
    else
      sprintf "%A is a invalid latitude value" lat |> Error

Longitude

type Longitude = private Longitude of float with
  // float
  member this.Value = let (Longitude lng) = this in lng
  // float -> Result<Longitude, string>
  static member TryCreate (lng : float) =
    if lng >= -180. && lng <= 180. then
      Ok (Longitude lng)
    else
      sprintf "%A is a invalid longitude value" lng |> Error

Location

type Location =
  { Latitude : Latitude
    Longitude : Longitude }
  static member Create lat lng =
    { Latitude = lat; Longitude = lng }

Then, we can use the Result.map2 function as below to create the location with validation:

let validLatR = Latitude.TryCreate 13.067439
let validLngR = Longitude.TryCreate 80.237617

open FsToolkit.ErrorHandling

let result =
  Result.map2 Location.Create validLatR validLngR
(* Ok {Latitude = Latitude {Value = 13.067439;};
       Longitude = Longitude {Value = 80.237617;};} *)

When we try with an invalid latitude value, we'll get the following result:

let invalidLatR = Latitude.TryCreate 200.
let result =
  Result.map2 Location.Create invalidLatR validLngR
  // Error "200.0 is a invalid latitude value"