{-# OPTIONS_GHC -fglasgow-exts #-}

module Left where

import Generics.MultiRec.Base

class Left (phi :: * -> *) (f :: (* -> *) -> * -> *) where
    leftf :: phi ix -> (forall ix'. El phi ix' => phi ix' -> r ix') -> [f r ix]

instance (Left phi a, Left phi b) => Left phi (a :+: b) where
leftf w f = map L (leftf w f) ++ map R (leftf w f)

instance (Constructor c, Left phi f) => Left phi (C c f) where
leftf w f = map C (leftf w f)

instance (Left phi a, Left phi b) => Left phi (a :*: b) where
    leftf w f = zipWith (:*:) (leftf w f) (leftf w f)

instance (El phi xi) => Left phi (I xi) where
    leftf _ f = [I (f index)]

instance Left phi U where
    leftf _ _ = [U]

instance (EqS phi, El phi ix, Left phi f) => Left phi (f :>: ix) where
    leftf w f =
        case eqS (proof :: phi ix) w of
            Nothing -> []
            Just Refl -> map Tag (leftf w f)

instance LeftA a => Left phi (K a) where
    leftf _ _ = [K lefta]

class LeftA a where
    lefta :: a

instance LeftA Char where
    lefta = 'L'

instance LeftA () where
    lefta = ()

left :: (El phi ix, Fam phi, Left phi (PF phi)) => phi ix -> ix
left w = to w $ head $ leftf w (I0 . left)

On Mon, May 25, 2009 at 15:11, Sebastiaan Visser <sfvisser@cs.uu.nl> wrote:

Hey guys,

While playing around with MultiRec most things are pretty straightforward and most usage can be derived from the examples. The only thing that seems tricky to do, and of which I cannot find any examples, are generic producers.

Any examples of generic producers that take no values as input but do produce values as output? E.g. generic parsers (read), binary get, arbitrary?

I have some `SingleRec' producers that are fairly trivial to port except for the `Tag' case. Any clues?

Thanks,

--
Sebastiaan Visser

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