On Thu, Dec 11, 2008 at 09:49, Neil Mitchell <ndmitchell@gmail.com> wrote:

Hi Pedro,

>> I'm not sure there is a nice solution - reflection at the type level
>> (using _|_ at the value level), combined with strictness at the value
>> level, has limitations. It may be that the reflection machinery in SYB
>> can be tweaked to either alert the user in advance (i.e. by getting
>> the strictness of various fields), or providing some operation
>> combining gmapQ and fromConstr which isn't strict. To see my use case
>> take a look at "contains" in:
>>
>> http://www.cs.york.ac.uk/fp/darcs/uniplate/Data/Generics/PlateData.hs
>
> I'm not sure there's an easy solution either. As you say, the problem here
> seems to be caused by the strictness. Getting the strictness of each field
> would require changes to the representation types and to the deriving
> mechanism.
>
> Would your problem be solved if you used fromConstrB instead of simply
> fromConstr and built an entirely determined (without bottoms) value?

I might be able to use fromConstrB, but it requires a lot more work -
initialising lots of things and creating lots of dummy values. I'll
look into it.

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

I also note that the documentation for fromConstrB seems to have
disappeared. See:
http://haskell.org/ghc/docs/latest/html/libraries/syb/doc-index.html -
the entry is still there but the link is gone.

fromConstrB is not in the syb package. It's in base4, in Data.Data: http://www.haskell.org/ghc/dist/stable/docs/libraries/base/Data-Data.html#v%3AfromConstrB

Thanks,
Pedro

(I'm also aware that the Hoogle documentation for it is missing, but
hope to fix that this weekend - I've had issues trying to build
things)

Thanks

Neil