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elm

Generic pattern matching in Elm

I'm trying to build a generic mapping function that will extract a possible inner type from a union type, apply a transform to the inner value and map it back to the outer type.

The trouble is, I need a way to distinguish if a specific value of the outer type has an inner type at all.

It would work for me if the code below actually compiled, but Elm doesn't allow constants in pattern matches. (constructor on line 4 fails to compile)

Is there any other way to accomplish this?

map : (inner -> outer) -> (inner -> inner) -> outer -> outer
map constructor func current =
    case current of
        constructor child ->
            constructor (func child)

        _ ->
            current
Solution

  • No, unfortunately Elm doesn't have a way to express this kind of thing generically, you need to write a specific version for each custom type.