[Haskell-cafe] simple generic / data view library?

[José Pedro Magalhães]

Hello Conal,

What you've done looks very much like the Regular datatype [1] in the
rewriting library [2]. The rewriting library, as its name indicates, is very
much targeted at rewriting. For a more complete library using a sum of
products view (and without type synonyms), try the new release of EMGM [3].


Cheers,
Pedro

[1]
http://hackage.haskell.org/packages/archive/rewriting/0.1/doc/html/Generics-Regular-Rewriting-Representations.html#t%3ARegular
[2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rewriting
[3] http://www.cs.uu.nl/wiki/bin/view/GenericProgramming/EMGM

2008/11/18 Conal Elliott <conal at conal.net>

> Is there a simple existing library that provides views of data types in
> terms of unit, product and sum?
>
> Here's what I threw together for my own use.  I used associated types,
> though functional dependencies would work as well.
>
>     class HasView t where
>       type View t
>       view   :: t -> View t
>       unview :: View t -> t
>
>     -- View instances
>
>     instance HasView (Maybe a) where
>       type View (Maybe a) = Either () a
>
>       view   Nothing      = (Left ())
>       view   (Just a)     = (Right a)
>
>       unview (Left ())    = Nothing
>       unview (Right a)    = (Just a)
>
>
>     instance HasView [a] where
>       type View [a]         = Either () (a,[a])
>
>       view   []             = (Left  ())
>       view   (a:as)         = (Right (a,as))
>
>       unview (Left  ())     = []
>       unview (Right (a,as)) = (a:as)
>
>
>     onView2 :: (HasView a, HasView b, HasView c) =>
>            (View a -> View b -> View c)
>         -> (a -> b -> c)
>     onView2 op a b = unview (view a `op` view b)
>
> Thanks,  - Conal
>
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