Statically-typed stack effects

gistfile1.hs

module Stack where

import Control.Arrow

class List a

-- Use pairs to represent a cons list which can be statically checked
instance List ()
instance List b => List (a, b)

safeHead :: List b => (a, b) -> a
safeHead = fst

safeTail :: List b => (a, b) -> b
safeTail = snd

-- Given an element, creates a stack effect which pushes it
push :: List b => a -> b -> (a, b)
push = (,)

-- Lifts a 2-argument function to work on stacks
bop :: List l => (a -> b -> c) -> (b, (a, l)) -> (c, l)
bop f (x, (y, z)) = (f y x, z)

swap :: List c => (a, (b, c)) -> (b, (a, c))
swap (x, (y, z)) = (y, (x, z))

main :: IO ()
main = print $ push 1         >>>
               push 2         >>>
               swap           >>>
               push 3         >>>
               bop (+)        >>>
               swap           >>>
               bop div        >>>
               push "Hello, " >>>
               push "world!"  >>>
               bop (++) $ ()