Untyped Elm Programs (oh no not again!)

[rupert]

Topic is closed, so creating a new thread…

Can an Elm program ever be untyped?

Elm has polymorphism, but we usually cannot create polymorphic values. An example: foldr : (a -> b -> b) -> b -> List a -> b To call this, I need a list of something: foldr (\w acc -> w ++ " " ++ acc) "" ["words", "to", "join", "with", "spaces"] Its polymorphic, but I needed to give it some values to do anything useful, and that means that the program ended up fully typed at runtime. Ok, I don’t have to give it concrete values immediately, the polymorphism could continue up the call stack as…

testValue : List a
testValue =
      let
          process xs = case xs of
              [] -> []
              nonEmpty -> transform nonEmpty

          transform xs = process xs
      in
      process [1,2,3]

An example of interest (to me anyway!). This produces a polymorphic value. Except of course that this program never terminates and yields the value. So it still will not actually give us an untyped value in Elm. But it is interesting to consider from a compilation perspective because the compiler still has to be able to compile and run such programs.