endo

Endo for Elm: Endomorphisms to simplify code

You Don't Need This Package

But you can use it if you want

This mostly exists to show a pattern since this package is only one thing.

type alias Endo a = a -> a

So why is this useful? Well for one it can make type signatures more concise, but more importantly, it lets you know that the type didn’t change. Let’s look at an example of common code:

type alias P = { x : Int, y : Int }

type alias Point = Point P

coordinates : List Point
coordinates =
    [ Point { x = 0, y = 0 }
    , Point { x = 1, y = 2 }
    , Point { x = 8, y = 4 }
    ]

So one task might be that we want to reset all of the x coordinates to 0. Normally we’d do this with List.map.

setX : a -> { r | x : a } -> { r | x : a }
setX x_ r = { r | x = x_ }

mapPoint : (P -> P) -> Point -> Point
mapPoint fn (Point p) = Point (fn p)

newCoordinates : List Point
newCoordinates = List.map (mapPoint (setX 0)) coordinates

That’s fine and it works, but it’s a little verbose and when mapping a Point we really want to ensure that cannot modify the types along the line. Because Elm frowns upon the use of optics, a map function is usually used as a setter of sorts and a setter shouldn’t change the types.

So if we go back to Endo, you can see that we can say these types do not change.

type alias Endo a = a -> a

setX : a -> Endo { r | x : a }
setX x_ r = { r | x = x_ }

mapPoint : Endo P -> Endo Point
mapPoint fn (Point p) = Point (fn p)

newCoordinates : List Point
newCoordinates = List.map (mapPoint (setX 0)) coordinates

So, you can pull this package in … or not, and just make you own Endo in your project 🤷.