[Haskell-cafe] [] == []

[Paul Keir]

f''' = ([]::[()]) == ([]::[()])

(Very pretty.)

So why doesn't ghc have 'default' instances?

-----Original Message-----
From: Eugene Kirpichov [mailto:ekirpichov at gmail.com]
Sent: Fri 29/05/2009 10:51
To: Paul Keir
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] [] == [][MESSAGE NOT SCANNED]
 
2009/5/29 Paul Keir <pkeir at dcs.gla.ac.uk>:
> 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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>
>



-- 
Eugene Kirpichov
Web IR developer, market.yandex.ru


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