[Haskell-cafe] Haskell-Cafe Digest, Vol 174, Issue 13

[Mikhail Baykov]

recursion-schemes package might do the trick. I wrote TH for it and
somebody took care of getting it merged in. It should simplify some of
 this boilerplate.

>
> I am looking for a solution to get rid of this silly boilerplate:
>
> eval :: Ord var => Map var Bool -> Proposition var -> Bool
> eval ctx prop = evalP $ fmap (ctx Map.!) prop
>   where
>     evalP = \case
>         Var b -> b
>         Not q -> not $ evalP q
>         And p q -> evalP p && evalP q
>         Or p q -> evalP p || evalP q
>         If p q -> evalP p ==> evalP q
>         Iff p q -> evalP p == evalP q
>
> What I would like to do in essence is to replace the data constructors like so:
>
> -- Not valid Haskell!! Can't pattern match on constructor only...
> magic = \case
>     Var -> id
>     Not -> not
>     And -> (&&)
>     Or -> (||)
>     If -> (==>)
>     Iff -> (==)
>
> compile = transformAST magic $ fmap (\case 'P' -> False; 'Q' -> True)
>
>>>> compile (Iff (Not (And (Var 'P') (Var 'Q'))) (Or (Not (Var 'P')) (Not (Var 'Q'))))
>             ((==) (not ((&&) (id True) (id False))) ((||) (not (id True)) (not (id False))))
>
> Note how the compiled expression exactly mirrors the AST, so there should be some meta programming technique for this.
>
> Does anyone have an idea how I can achieve this?
>
> The full source code is here: https://gist.github.com/vimuel/7dcb8a9f1d2b7b72f020d66ec4157d7b