map2

Result.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 =
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 > -180. && lat <= 180. 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 >= -90. && lng <= 90. 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"