New subject: Haskell-Cafe Digest, Vol 174, Issue 13

13 Feb 2018


      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

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

Mikhail Baykov

Doaitse Swierstra

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