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

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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