f''' = ([]::[()]) == ([]::[()])
(Very pretty.)
So why doesn't ghc have 'default' instances?
-----Original Message-----
From: Eugene Kirpichov [mailto:ekirpichov@gmail.com]
Sent: Fri 29/05/2009 10:51
To: Paul Keir
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] [] == [][MESSAGE NOT SCANNED]
2009/5/29 Paul Keir
Hi all,
GHC is not happy with this:
f = [] == []
This fails because GHC doesn't know which 'a' you mean, and can't choose an Eq instance.
nor this:
f' = ([]::(Eq a) => [a]) == ([]::(Eq a) => [a])
This fails for the same reason.
but this is OK:
f'' = ([]::[Integer]) == ([]::[Integer])
GHCI is comfortable with [] == [], so why not GHC? 'Just curious.
Because GHCI has some 'default' instances, whereas GHC doesn't. This time, it probably chooses a=().
Cheers, Paul
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