GADTs and Scrap your Boilerplate
- by finnsson
I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (at http://github.com/finnsson/Text.XML.Generic) and so far I got working code for "normal" ADTs but I'm stuck at deserializing GADTs.
I got the GADT
data DataBox where
DataBox :: (Show d, Eq d, Data d) => d -> DataBox
and I'm trying to get this to compile
instance Data DataBox where
gfoldl k z (DataBox d) = z DataBox `k` d
gunfold k z c = k (z DataBox) -- not OK
toConstr (DataBox d) = toConstr d
dataTypeOf (DataBox d) = dataTypeOf d
but I can't figure out how to implement gunfold for DataBox.
The error message is
Text/XML/Generic.hs:274:23:
Ambiguous type variable `b' in the constraints:
`Eq b'
arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29
`Show b'
arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29
`Data b' arising from a use of `k' at Text/XML/Generic.hs:274:18-30
Probable fix: add a type signature that fixes these type variable(s)
It's complaining about not being able to figure out the data type of b.
I'm also trying to implement dataCast1 and dataCast2 but I think I can live without them (i.e. an incorrect implementation).
I guess my questions are:
Is it possible to combine GADTs with Scrap your Boilerplate?
If so: how do you implement gunfold for a GADT?