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 existentials.
I got the existential data type
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 existentials with Scrap your Boilerplate?
- If so: how do you implement gunfold for an existential data type?