generics - Data.Data -- producing dataCast1 for an arity 2 type constructor (partially specialized) -


so data.map has datacast2 defined, makes sense, has arity 2 type constructor. datacast1 defaults const nothing. datacast2 defined gcast2.

for reference:

class typeable => data     datacast1 :: typeable1 t => (forall d. data d => c (t d)) -> maybe (c a)     datacast2 :: typeable2 t => (forall d e. (data d, data e) => c (t d e)) -> maybe (c a)     ...  gcast1 :: (typeable1 t, typeable1 t') => c (t a) -> maybe (c (t' a)) gcast2 :: (typeable2 t, typeable2 t') => c (t b) -> maybe (c (t' b))

the question @ hand this: given in data.data, data.typeable, etc., , given arity 2 type constructor datacast2 defined (say, map, or (,) ), possible write version of datacast1 right thing partial specialization of type constructor, either 1 specific constructor @ time, or in general?

intuitively, think there should solution, first few tries crashed , burned.

i'm not sure if want, might steer in right direction if it's not. written in similar style gcast, gcast1, , gcast2 functions in data.typeable library. more details, "read source, luke".

mydatacast1 :: forall c t d e a.(typeable d, typeable e) => c (t d a) -> maybe (c (t e a)) mydatacast1 x = r         r = case typeof (getarg x) == typeof (getarg (fromjust r)) of            true  -> $ unsafecoerce x            false -> nothing      getarg :: c (t x a) -> x      getarg = undefined

using function can, instance write foo

foo :: typeable d => c (d, a) -> maybe (c (int, a)) foo = mydatacast1

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