[Haskell-cafe] different behaviours with or without putStrLn

[Carter Schonwald]

Try

*instance NFData a => NFData (MyGADT (Maybe a) )*

On Sunday, January 19, 2014, Corentin Dupont <corentin.dupont at 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 at gmail.com<javascript:_e({}, 'cvml', 'allbery.b at gmail.com');>
> > wrote:
>
>> On Sun, Jan 12, 2014 at 6:44 PM, Corentin Dupont <
>> corentin.dupont at gmail.com <javascript:_e({}, 'cvml',
>> 'corentin.dupont at 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 at gmail.com <javascript:_e({}, 'cvml', 'allbery.b at gmail.com');>
>> ballbery at sinenomine.net <javascript:_e({}, 'cvml',
>> 'ballbery at sinenomine.net');>
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>>
>
>
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