同じ関数に違う数の引数を指定した関数とかを、一つの演算子に適用する感じ。

gistfile1.hs

{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleInstances #-}
module Main where

data Seq where
  Seq :: (Sequensable a, Sequensable b) => a -> b -> Seq
data Note = Note { fromNote :: Int } deriving Show

a :: Int -> Int -> Note
a x y = Note $ x + y

b :: Int -> Int -> Note
b x y = Note $ x * y

---

class Sequensable s where
  toNotes :: s -> [Note]

instance Sequensable Note where
  toNotes = (:[]).id
instance Sequensable (Int -> Note) where
  toNotes f = [f 0]
instance Sequensable (Int -> Int -> Note) where
  toNotes f = [f 0 0]
instance Sequensable Seq where
  toNotes (Seq x xs) = toNotes x ++ toNotes xs

-----

infixr 9 ==>
(==>) :: (Sequensable a, Sequensable b) => a -> b -> Seq
(==>) = Seq

-- a が足し算、 b が掛け算、指定されなかった引数には 0 が適用される
test2 :: Seq
test2 = a ==> b ==> a 5 ==> b 5 ==> a 1 2 ==> b 2 3 

-----
-- test

main :: IO ()
main = print . map fromNote . toNotes $ test2 -- [0, 0, 5, 0, 3, 6]