On 2 February 2011 02:25, Sebastian Fischer
It's a nice trick! Although it does look strange, it may be reasonable to allow pattern bindings in instance declarations regardless of the original proposal. Is it correct that, currently, pattern bindings are allowed everywhere but in instance declarations? If so, why not in instance declarations too?
Unfortunately, they are not allowed. I ran up against this limitation even before this thread. Here is an example: """ {-# LANGUAGE NoMonoPatBinds #-} data I a = I { unI :: a } instance Monad I where (return, (>>=)) = (I, \mx f -> f (unI mx)) main = return () """ And the error: """ /Users/mbolingbroke/Junk/InstancePattern.hs:5:5: Pattern bindings (except simple variables) not allowed in instance declarations (return, >>=) = (I, \ mx f -> f (unI mx)) """ This behaviour does seem to be as per the Haskell 98 spec, but I'm not sure of the motivation behind it.
I think the proposals to make pattern bindings monomorphic only concern pattern bindings without type annotations. Instance methods do have type annotations in the class declaration so even if pattern bindings without type signatures would be monomorphic, instance methods bound using pattern bindings need not be.
This was what I used to think too, but I recently found out (again, unrelated to this thread) that they won't work even with explicit signatures! See for example http://hackage.haskell.org/trac/ghc/ticket/4940 I'm personally not in favour of MonoPatBinds anyway, but it is *particularly* annoying that they don't work even with a signature. Cheers, Max