What you say certainly sounds plausible, although I do not have DataRep paged into my brain at the moment.  If the generics folk are happy, do make a libraries proposal and, after discussion period, execute!

Simon

From: generics-bounces@haskell.org [mailto:generics-bounces@haskell.org] On Behalf Of José Pedro Magalhães
Sent: 08 December 2008 12:33
To: Generics Mailing List
Cc: Americo Vargas
Subject: [Hs-Generics] SYB's StringRep behavior

Hello,

SYB uses DataRep to represent datatypes:

-- | Public representation of datatypes
data DataRep = AlgRep [Constr]
             | IntRep
             | FloatRep
             | StringRep
             | NoRep

Am I right to believe that StringRep should be CharRep? Note that IntRep is used for the primitives Int and Integer datatypes, FloatRep for Float and Double, and StringRep (apparently) for Char. String, however, is represented as 'AlgRep [[],(:)]':

*Main> dataTypeOf 'p'
DataType {tycon = "Prelude.Char", datarep = StringRep}
*Main> dataTypeOf "p"
DataType {tycon = "Prelude.[]", datarep = AlgRep [[],(:)]}

This makes sense, since String is not a primitive datatype. But it causes the apparently wrong behavior:

*Main> fromConstr (mkStringConstr (dataTypeOf "a") "ab") :: String
"*** Exception: mkStringConstr
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "ab") :: String
"*** Exception: constrIndex

The correct way of using mkStringConstr is to construct a Char. This, however, only works for strings with a single character:

*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "b")  :: Char
'b'
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "ab") :: Char
*** Exception: gunfold
*Main> fromConstr (mkStringConstr (dataTypeOf 'a') "")   :: Char
*** Exception: gunfold

Wouldn't it be more clear if StringRep would be named CharRep and mkStringConstr named mkCharConstr?

Thanks,
Pedro