The idea that wanting to use typeclasses in Elm is an impedance mismatch would be more convincing if Elm and its standard library truly did everything without them, using alternate approaches like explicit dictionary passing. But as you probably know, they don’t. Instead, the language has special syntax for three builtin “typeclasses”: comparable, appendable, and number. And the standard library uses them: its Dict, for example, requires the key type to be comparable. These classes are automatically implemented for various builtin types, but there’s no way to implement them for custom types, so you can’t have Dicts with custom types as keys (unless that changed very recently). And if you want to, say, implement a hash map data structure, which requires the keys to be hashable, you have to use a completely different design, since “hashable” is not one of the three magic typeclasses.
To me, rather than principled, that just feels incomplete.
