Re: [Haskell-cafe] different behaviours with or without putStrLn

20 Jan 2014

      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  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 
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 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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Re: [Haskell-cafe] different behaviours with or without putStrLn

Andras Slemmer