How to declare multiple Literals with only one Literal attribute in F#?

Question

I am trying to find an elegant way of assigning keys to symbols without having to do something like the following.

let [<Literal>] North = ConsoleKey.UpArrow // etc.

I'd rather do something like this, using only one attribute. Is there any way I can do that?

[<Literal>]
type Direction =
    | North of ConsoleKey.UpArrow
    | East of ConsoleKey.RightArrow
    | South of ConsoleKey.DownArrow
    | West of ConsoleKey.LeftArrow

Answer 1

Assuming your aim is to use these in a pattern match, here is one way to do it:

// Use a type alias to shorten the name for ConsoleKey
type Key = ConsoleKey

// Create a general purpose active pattern that simply tests for equality
let (|Is|_|) a b = if a = b then Some () else None

// This is how you would use it
let describeMovement key =
    match key with
    | Is Key.UpArrow -> "up"
    | Is Key.RightArrow -> "right"
    | Is Key.DownArrow -> "down"
    | Is Key.LeftArrow -> "left"
    | _ -> "invalid"