Deniz Dogan wrote:

...

instance Num Bool where (+) False = id (+) True = not

(*) True True = True (*) _ _ = False

Isn't "XOR" for booleans (/=)?

Oh right. And (*) would be (&&): instance Num Bool where (+) = (/=) (*) = (&&) -- ... //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

Err, I'm not seeing the danger of this (+) :: forall a. (Num a) => a -> a -> a Doesn't this require the two parameters to be the same instance of Num? On Fri, May 8, 2009 at 10:51 AM, Sittampalam, Ganesh < ganesh.sittampalam@credit-suisse.com> wrote:

Stephan Friedrichs wrote:

...

When looking for an xor function, I found one in Data.Bits but couldn't use it for Bool, because Bool is no instance of Bits and of Num (which would be necessary, because it's "class (Num b) => Bits b"). My question is: Why not?

[...] quite trivial... Why is this not part of base? Or am I missing something?

One reason would be that we don't want 1 + True to typecheck, even if it does have a sensible interpretation.

Ganesh

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Hmm, I never knew that. Is that a GHC thing? Is it strictly necessary? Seems like it could be done in the Num instance for Integers, Ints, etc. On Fri, May 8, 2009 at 11:51 AM, Neil Mitchell wrote:

...

Err, I'm not seeing the danger of this (+) :: forall a. (Num a) => a -> a -> a Doesn't this require the two parameters to be the same instance of Num?

I didn't at first, then I remembered:

1 + True = fromInteger 1 + True

And if we have Num for Bool, it type checks.

Thanks

Neil

Neil Mitchell wrote:

I didn't at first, then I remembered:

1 + True = fromInteger 1 + True

And if we have Num for Bool, it type checks.

Does that also mean that you could write: if 3 - 4 then ... else ... (= if (fromInteger 3 :: Bool) - (fromInteger 4 :: Bool) then ... else ...) or perhaps (not sure if type defaulting stretches to this): if 1 then ... else ... (= if (fromInteger 1 :: Bool) then ... else ...) If you change fromInteger in Num Bool to be fromInteger x = x /= 0, then we could all start writing nasty C-like if-expressions... Thanks, Neil.

...

Does that also mean that you could write:

if 3 - 4 then ... else ...  (= if (fromInteger 3 :: Bool) - (fromInteger 4 :: Bool) then ... else ...)

No. 3 - 4 is an Integer, the proposal is to convert Bools to Ints, not Ints to Bools.

Rather, (3 - 4) is a "(Num t) => t", so yes, this would work with instance Num Bool. *Main> if 3 - 4 then "yessirree" else "yep" "yep"

Yeah, the more people give examples of the power of Num Bool, the more it seems like a very bad idea!

+1 John

On May 8, 2009, at 12:00 , Neil Brown wrote:

If you change fromInteger in Num Bool to be fromInteger x = x /= 0, then we could all start writing nasty C-like if-expressions...

I'd be strongly tempted to say

fromInteger = const False

-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

"[...] unsafe [PerformIO ...]" looks safer to me.

Hmmm.... Stefan

Agreed. I wound up having to add a horrible Num instance for Bool in 'monoids' in order to support a decent Boolean Ring type. http://comonad.com/haskell/monoids/dist/doc/html/monoids/Data-Ring-Boolean.h... I would much rather be able to get rid of it! The only problem with eliminating the constraint is that any code that uses Bits polymorphically might have to pick up a Num annotation, but I can't see it being a serious problem. -Edward Kmett On Wed, Jun 24, 2009 at 8:13 AM, John Meacham wrote:

On Fri, May 08, 2009 at 04:36:41PM +0200, Stephan Friedrichs wrote:

...

When looking for an xor function, I found one in Data.Bits but couldn't use it for Bool, because Bool is no instance of Bits and of Num (which would be necessary, because it's "class (Num b) => Bits b"). My question is: Why not?

This has bothered me too. However, I think the root problem is that 'Num' is a superclass of 'Bits'. There is no reason it should be, all the default instances can be specified without the Num dependency.

John

-- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe