On Friday, March 14, 2014 5:19:40 PM UTC-4, Eric Walkingshaw wrote:
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
That's a great point, thanks. I'll have to remember to interrogate myself on that next time I find myself reaching for this pattern. I wonder if your version can be made to work for functions of any arity? But in most (maybe not all) cases I really don't want to be defining new types. Thanks, Brandon