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