Off the top of my head i can think of two non-overlapping solution, both are ugly but maybe someone has an idea to make one nice: The first is to bake the constraint into the GADT: data MyGADT a where MyField :: NFData b => b -> MyGADT (Maybe b) instance NFData (MyGADT a) where rnf (MyField b) = rnf b This is bad because NFData should have nothing to do with the definition of the datastructure The second is to use implication constraints: import Data.Constraint data MyGADT a where MyField :: b -> MyGADT (Maybe b) newtype Constr a = Constr (forall b. (a ~ Maybe b) :- NFData b) class DConstr a where constr :: Constr a instance (NFData a) => DConstr (Maybe a) where constr = Constr (Sub Dict) instance (DConstr a) => NFData (MyGADT a) where rnf (MyField b) = case constr :: Constr a of Constr (Sub Dict) -> rnf b this is ugly because for every non-Maybe index you need to define an "absurd" instance of DConstr. e.g. for Int: instance DConstr Int where constr = Constr undefined -- should be (Constr (Sub !absurd!)) but there is no way to denote absurdity in Haskell On 19 January 2014 23:16, Bob Ippolito <bob@redivi.com> wrote:
For that to work, I believe you must enable FlexibleInstances and possibly also OverlappingInstances (depending on what the rest of the GADT looks like).
On Sun, Jan 19, 2014 at 3:00 PM, Carter Schonwald < carter.schonwald@gmail.com> wrote:
Try
*instance NFData a => NFData (MyGADT (Maybe a) )*
On Sunday, January 19, 2014, Corentin Dupont <corentin.dupont@gmail.com> wrote:
Hi, I've tried the make an instance of NFData for all types, although I have a problem with the GATD:
*data MyGADT a where MyField :: b -> MyGADT (Maybe b) instance NFData a => NFData (MyGADT a) where rnf (MyField b) = (rnf b)* Gives me : Could not deduce (NFData b) arising from a use of `rnf' from the context (NFData a)
Which I understand (that's because a ~ *Maybe b *and not* a ~ b *due to the declaration of *MyField b *being of type *MyGADT (Maybe b)*)
*But I have no idea how to solve that?? *
*Thanks...*
*Corentin*
On Mon, Jan 13, 2014 at 1:10 AM, Brandon Allbery <allbery.b@gmail.com>wrote:
On Sun, Jan 12, 2014 at 6:44 PM, Corentin Dupont < corentin.dupont@gmail.com> wrote:
One question: Since it works with putStrLn, can I simulate the behaviour of putStrLn without actually... printing anything? As a workaround...
Worst case, open a handle on /dev/null and hPutStr to it.
I'd probably try to figure out how to write appropriate NFData instances, though.
-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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