[Hs-Generics] RE: Generics

[José Pedro Magalhães]

On Thu, Dec 11, 2008 at 09:57, Neil Mitchell <ndmitchell at gmail.com> wrote:

> HI Pedro,
>
> > Would this help?
> >
> >> {-# LANGUAGE ScopedTypeVariables  #-}
> >> {-# LANGUAGE FlexibleContexts     #-}
> >>
> >> module Data.Generics.Builders (empty) where
> >>
> >> import Data.Data
> >> import Data.Generics.Aliases (extB)
> >>
> >> -- | Construct the empty value for a datatype. For algebraic datatypes,
> >> the
> >> -- leftmost constructor is chosen.
> >> empty :: forall a. Data a => a
> >> empty = general
> >>       `extB` char
> >>       `extB` int
> >>       `extB` integer
> >>       `extB` float
> >>       `extB` double where
> >>   -- Generic case
> >>   general :: Data a => a
> >>   general = fromConstrB empty (indexConstr (dataTypeOf general) 1)
> >>
> >>   -- Base cases
> >>   char    = '\NUL'
> >>   int     = 0      :: Int
> >>   integer = 0      :: Integer
> >>   float   = 0.0    :: Float
> >>   double  = 0.0    :: Double
>
> Yep, that should do it!


It's slated for release with the next version of the SYB library, so in a
near future you'll only need to import it.


Cheers,
Pedro
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