[Haskell-cafe] translating from fundeps into type families
Manuel M T Chakravarty
chak at cse.unsw.edu.au
Tue Apr 8 22:29:19 EDT 2008
> 2008/4/8, Manuel M T Chakravarty <chak at cse.unsw.edu.au>:
>> You need to write the instance as
>> instance (b ~ TheFoo a, Foo a) => Bar (Either a b) where
>> bar (Left a) = foo' a
>> bar (Right b) = foo' (foo b :: a)
> If you do that, the program compile, but res still raise a panic in
> GHC6.8.2 .
> Before you sent your solution, I also tried to do
> class (TheFoo a ~ b) => Foo a b where ...
> instance (Foo a b) => Bar (Either a b) where ...
> But it didn't compile, it seems to me both versions should have the
> same behaviour ?
6.8.2 has many bugs concerning type families that have been removed in
the development version of GHC.
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