[Haskell-cafe] I seem to constantly abuse TypeFamilies; what do i really want?

[Jake McArthur]

A benefit of using type families and type classes instead of GADTs for this
kind of thing when you can is they are usually cheaper. You can often write
code that inlines perfectly with former but ends up being some recursive
function that will never inline with the latter.

- Jake
On Mar 14, 2014 5:20 PM, "Eric Walkingshaw" <walkiner at eecs.oregonstate.edu>
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
>
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