[Haskell-cafe] Replace data constructors via meta programming

[David Fox]

You actually can pattern match on constructor only:

magic = \case
    Var {} -> id
    Not  {}-> not
    And {} -> (&&)
    Or {} -> (||)
    If {} -> (==>)
    Iff {} -> (==)

On Sun, Feb 11, 2018 at 6:30 PM, Vilem-Benjamin Liepelt <vl81 at kent.ac.uk>
wrote:

> Hi,
>
> 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
>
> I am happy to take any other comments relating to my code...
>
> Best,
>
> Vilem
>
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