Re: Use sharing in the Alternative instance of Maybe

On Wed, Dec 14, 2011 at 1:49 PM, Bas van Dijk wrote:

Hello,

When looking at the Alternative instance of Maybe I noticed that the <|> combinator doesn't share it's first argument when it could. So I propose to make the following change:

 instance Alternative Maybe where     empty = Nothing -    Nothing <|> p = p -    Just x <|> _ = Just x +    Nothing <|> r = r +    l       <|> _ = l

Since this isn't a change in the API, I guess this doesn't need to go through the library submission process.

http://hackage.haskell.org/trac/ghc/ticket/5699

Cheers,

Bas

+1 Out of curiosity, does this actually bypass an extra allocation, or is GHC smart enough to notice the possibility to share? Michael