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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On Fri, May 29, 2009 at 5:36 AM, Max Rabkin wrote:

On Fri, May 29, 2009 at 12:29 PM, Paul Keir wrote:

...

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

(Very pretty.)

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

It does. I believe Num defaults to Integer and then to Double.

Generally, though, defaults are convenient in exploratory usage but confusing in compiled code. In compiled code, you don't want arbitrary choices of defaults to affect performance and correctness.

I've had programs run much slower than expected because the types defaulted to Integer rather than Int.

http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluati...