Working through the Operational Monad Tutorial

Program.hs

{-# LANGUAGE GADTs #-}
module Program where
--
-- The Operational Monad Tutorial:
-- http://apfelmus.nfshost.com/articles/operational-monad.html
--

-- GADT
data Program instr a where
    Then   :: instr a -> (a -> Program instr b) -> Program instr b
    Return :: a -> Program instr a

type StackProgram a = Program StackInstruction a

data StackInstruction a where
    -- Swap :: StackInstruction ()
    Push :: Int -> StackInstruction ()
    Pop  :: StackInstruction Int

type Stack a = [a]

interpret :: StackProgram a -> Stack Int -> a
interpret (Then (Push a) is) stack     = interpret (is ()) (a:stack)
-- interpret (Push a `Then` is) stack     = interpret (is ()) (a:stack)
interpret (Pop    `Then` is) (b:stack) = interpret (is b ) stack
interpret (Return c)         stack     = c

-- interpret :: StackProgram -> Stack Int -> Stack Int
-- interpret (Push a : is) stack = interpret is (a : stack)
-- interpret (Pop    : is) stack = interpret is (tail stack)
-- interpret []            stack = stack

-- example :: StackProgram
-- example = Push 5 : Push 42 : Pop : []

-- example2 = do
--   a <- pop   -- Pop `Then` \a ->
--   b <- pop   -- Pop `Then` \b ->
--   push (a+b) -- Push (a+b) `Then` Return

example2 = Pop `Then` \a ->
           Pop `Then` \b ->
           Return (a+b)
           -- Push (a+b) `Then` Return
           -- Push (a+b) `Then` \_ -> Pop `Then` \c -> Return (c*10)

example3 = Pop `Then` \a -> Pop `Then` \b -> Return (a*b)

-- f :: String -> Int -> String
-- f s n = s ++ show n

-- prefixWithABC :: Int -> String
-- prefixWithABC = f "ABC"

-- f' :: String -> (Int -> String)
-- f' s =
--   where
--     g n = s ++