[Haskell-cafe] Replace data constructors via meta programming
David Fox
dsf at seereason.com
Thu Feb 15 13:51:00 UTC 2018
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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