Question

This question arose while reading the new chapter in the excellent Learn You a Haskell about applicative functors.

The Applicative typeclass has, as part of the definition for the Maybe instance:

pure = Just

If I just go to GHCi and import Control.Applicative, and do:

pure (3+)

I don’t get Just anything (makes sense). But if I use it in part of the expression:

pure (3+) <*> Just 4

I get Just 7. I guess it also isn’t surprising, but I’m missing something integral about how typeclasses work, I think, that there is no ambiguity with the call to pure here.

If my confusion makes sense, can anyone explain what’s going on in detail?

Answer 1

It’s just type inference. The (<*>) operator requires both arguments to use the same Applicative instance. The right side is a Maybe, so the left side has to be a Maybe also. So that’s how it figures out which instance is used here. You can look at the type of any expression in the interpreter by typing :t expression, and maybe if you just go through each subexpression and look at the type that was inferred, you will get a better picture of what’s going on.