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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