On Fri, 14 Mar 2014 22:19:40 +0100, Eric Walkingshaw
I'm not sure if this answers your questions, but I think this particular problem has a cleaner solution with GADTs:
{-# LANGUAGE GADTs #-}
data Cmd s t where Push :: a -> Cmd s (a,s) F1 :: (a -> b) -> Cmd (a,s) (b,s) F2 :: (a -> b -> c) -> Cmd (a,(b,s)) (c,s)
data Prog s t where (:.) :: Cmd s t -> Prog t u -> Prog s u End :: Prog s s
infixr 5 :.
cmd :: Cmd s t -> s -> t cmd (Push a) s = (a, s) cmd (F1 f) (a,s) = (f a, s) cmd (F2 f) (a,(b,s)) = (f a b, s)
prog :: Prog s t -> s -> t prog (c :. p) s = prog p (cmd c s) prog End s = s
run :: Prog () t -> t run p = prog p ()
Then from GHCi:
> run (Push 3 :. Push 4 :. F2 (+) :. F1 show :. End) ("7",())
Maybe you really want GADTs? :)
-Eric
Of course, there's also:
start :: (() -> r) -> r start f = f ()
end :: (a, s) -> a end = fst
push :: s -> a -> ((a, s) -> r) -> r push s a f = f (a, s)
op2 :: (a -> b -> c) -> (a, (b, s)) -> ((c, s) -> r) -> r op2 o (a, (b, s)) f = f (a `o` b, s)
add, mul :: Num a => (a, (a, s)) -> ((a, s) -> r) -> r add = op2 (+) mul = op2 (*)
example :: Integer example = -- 35 start push 2 push 3 add push 7 mul end