Haskell-适用/单实例

Question

I've seen that the type of pure 1 is (Num a, Applicative f) => fa , which is quite obvious. So if I would like to make it a Maybe Int :

Prelude> pure 1 :: Maybe Int
Just 1

What about this?

Prelude> pure 1
1
Prelude> return 1
1

What is going on? Why doesn't it complain about not knowing which instance to choose??

Edit

I think that this behaviour actually has nothing to do with monads or applicatives, but this was the context I came to it...

Answer 1

That's a peculiarity of ghci. It has special handling for IO actions, which aren't just “computed” and printed (you can't print an IO action anyway), but executed and the result ^† printed. So,

Prelude> pure 1 :: IO Integer
1

As per behzad.nouri's comment, here's the relevant manual section .

---

^† _{Provided it's IO a with Show a and not a~() , otherwise only the action is executed.}