It would be a bit of a rabbit hole, as many common uses of typeclasses in Haskell require HKT, which Elm doesn't have. Typeclasses per se could be bolted on to

It would be a bit of a rabbit hole, as many common uses of typeclasses in Haskell require HKT, which Elm doesn't have. Typeclasses per se could be bolted on to Elm relatively easily, but I suspect that adding HKT without compromising Elm's other advantages (e.g. fast compiles, good error messages) would be quite tricky.