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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