[Haskell-cafe] different behaviours with or without putStrLn

[Bob Ippolito]

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 at gmail.com> wrote:

> 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>wrote:
>>
>>> On Sun, Jan 12, 2014 at 6:44 PM, Corentin Dupont <
>>> 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
>>> ballbery at sinenomine.net
>>> unix, openafs, kerberos, infrastructure, xmonad
>>> http://sinenomine.net
>>>
>>
>>
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