Question

After playing around with haskell a bit I stumbled over this function:

Prelude Data.Maclaurin> :t ((+) . ($) . (+))
((+) . ($) . (+)) :: (Num a) => a -> (a -> a) -> a -> a

(Data.Maclaurin is exported by the package vector-space.) So it takes a Num, a function, another Num and ultimately returns a Num. What magic makes the following work?

Prelude Data.Maclaurin> ((+) . ($) . (+)) 1 2 3
6

2 is obviously not a function (a->a) or did I miss out on something?

Answer 1

The Data.NumInstances module of the same package defines a Num instance for functions that return numbers:

instance Num b => Num (a->b) where
  (+)         = liftA2 (+)
  (*)         = liftA2 (*)
  fromInteger = pure . fromInteger
  ...

In Haskell an integer literal like 2 is generic so that it can represent a number for any instance of Num:

Prelude> :t 2
2 :: (Num t) => t

To convert it to an actual number of the type required in a specific context, fromInteger from the Num class is called.

Since the helper module mentioned above defines an instance of Num for functions, 2 can now be converted to a function with the fromInteger method specified there.
So ghci calls fromInteger 2 to get the function required as the second parameter of the construct in the question. The whole expression then happens to evaluate to 6.