Use sharing in the Alternative instance of Maybe

[basvandijk]

The <|> method of the Alternative instance of Maybe does not share its first argument when it could:

instance Alternative Maybe where
    empty = Nothing
    Nothing <|> p = p
    Just x  <|> _ = Just x

I propose to share this argument:

instance Alternative Maybe where
    empty = Nothing
    Nothing <|> r = r
    l       <|> _ = l

[igloo]

Applied:

commit 1d97ad10283d23b6dfb698a38ae57c29accd6ada
Author: Bas van Dijk <v.dijk.bas@gmail.com>
Date:   Wed Dec 14 12:24:38 2011 +0100

    Use sharing in the Alternative instance of Maybe