The Universe of Discourse

Fri, 26 Oct 2018

A snide addendum about implicit typeclass instances

In an earlier article I demanded:

Maybe someone can explain to me why this is a useful behavior, and then explain why it is so useful that it should happen automatically …

“This” being that instead of raising a type error, Haskell quietly accepts this nonsense:

   fmap ("super"++) (++"weasel")

but it clutches its pearls and faints in horror when confronted with this expression:

   fmap ("super"++) "weasel"

Nobody did explain this.

But I imagined someone earnestly explaining: “Okay, but in the first case, the (++"weasel") is interpreted as a value in the environment functor, so fmap is resolved to its the environment instance, which is (.). That doesn't happen in the second example.”

Yeah, yeah, I know that. Hey, you know what else is a functor? The identity functor. If fmap can be quietly demoted to its (->) e instance, why can't it also be quietly demoted to its Id instance, which is ($), so that fmap ("super"++) "weasel" can quietly produce "superweasel"?

I understand this is a terrible idea. To be clear, what I want is for it to collapse on the divan for both expressions. Pearl-clutching is Haskell's finest feature and greatest strength, and it should do it whenever possible.

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