Haskell 中的“f (a -> b)”类型签名是什么意思？

Question

I'm trying to understand applicatives in Haskell. Can't figure out what does following type signature mean:

f (a -> b)

For example:

foo :: Num a => Maybe (a -> a)
foo = Just (+1)

How can I understand the meaning of Maybe (a -> a) ? Is it a function? If it is, which types of arguments are allowed? Also obviously I'm new in functional programming, will be grateful for any resources on this topic.

Answer 1

In functional programming, functions are not so different from numbers or any other kind of value. Really the only difference is that the way you use a function is by applying it to an argument.

A value of type Maybe a is either the value Nothing or it is Just x , where x is of type a . So if you have a value of type Maybe (a -> a) , like your foo , it is either Nothing , or it is Just f where f is a function a -> a . In the least fancy way, you would use it like this:

case foo of
    Nothing -> "There was no function"
    Just f -> "There was a function, and its value at 0 is " ++ show (f 0)

So if it turns out that foo is not Nothing , then it contains Just a function as its value.

---

@Erich is right that especially the literal expression f (a -> b) is likely to be related to applicative functors, but that is not necessarily so. For example, a favorite type of mine is the type of isomorphisms -- equivalences between two types:

data Iso a b = Iso (a -> b) (b -> a)

Iso isn't even a Functor (a prerequisite of Applicative ), but it is still quite useful. It turns out that pairs are equivalent to functions from Bool . We could construct such an equivalence as an Iso value:

pairEquiv :: Iso (a,a) (Bool -> a)
pairEquiv = 
    Iso (\(x,y) -> \b -> if b then x else y) -- from pair to function
        (\f -> (f True, f False))            -- from function to pair

Here (Bool -> a) appears as an argument to a type constructor, and it just means that if you give the Iso a pair, it will give you a function back, and vice versa.

Answer 2

You can imagine f (a -> b) as a function of type a -> b wrapped in a context. It is mostly used in the context of Applicative s, that Maybe a is a prominent example for.

Applicative s are an extension of Functor s. A typical example for using Applicative s are functions with multiple arguments.

What if we have two Maybe Int s that we want to add up. We could try by partially applying + with fmap aka <$> . Thus we might try:

f :: Maybe (Int -> Int)
f = (+) <$> Just 3

But now, how do we apply this to a second argument. That's where we need the Applicative typeclass. It defines the <*> function. It has the type

<*> :: Applicative f => f (a -> b) -> f a -> f b

Thus we can use it to apply a second Maybe Int to our partially applied function f by doing:

> f <*> Just 4
Just 7

Without the helper function f the syntax resembles standard function application:

> (+) <$> Just 3 <*> Just 4
Just 7

For further reference see the chapter on applicative functors of learnyouahaskell.