[Haskell-cafe] Type system trickery

[Brandon S. Allbery KF8NH]

On Jun 22, 2009, at 14:43 , Andrew Coppin wrote:
> data Foobar a where
>   Foo :: X -> Y -> Foobar NoZoo
>   Bar :: X -> Y -> Foobar NoZoo
>   Zoo :: Foobar NoZoo -> Foobar Zoo
>
> For some reason, if I do this I get endless type check errors. I  
> have to change the top two back to Foobar a before it will work.  
> *sigh*


That's probably because ghc can't fix a type for Foobar a if you never  
actually use a anywhere.  Functional dependencies could solve that,  
but giving ghc a way to infer a type for it by using it where it  
doesn't affect anything important is easier and doesn't risk  
possible(?) weird interactions between FDs and GADTs.

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


-------------- next part --------------
A non-text attachment was scrubbed...
Name: PGP.sig
Type: application/pgp-signature
Size: 195 bytes
Desc: This is a digitally signed message part
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20090623/8cba3cb3/PGP.bin