[Haskell-cafe] closed classes

[Malcolm Wallace]

Ketil Malde <ketil+haskell at ii.uib.no> writes:

> Duncan Coutts <duncan.coutts at worcester.oxford.ac.uk> writes:
> 
> > closed class GConfValue v where
> 
> Hmm...doesn't 
> 
>     --8<--
>     module Closed(foo) where
>     class C a where foo = ...
>     instance C ...
>     --8<--
>     module Main where
>     import Closed
>     ...foo... 
>     --8<--
> 
> do what you want?  You can only use existing instances of C, but not
> declare them (outside of the Closed module), IIUC.

Ah, but now you cannot use (Closed t) => as a predicate in type
signatures, and since you cannot write a partial signature, you must
omit the signature altogether...

Regards,
    Malcolm