Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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.html 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 j...@repetae.net 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
Neil Mitchell wrote: [...] Which is a shame, having Bits on Bool seems entirely logical, having Num a superclass of Bits seems a little less clear. There are two default implementations in Bits bit i = 1 `shiftL` i x `testBit` i = (x .. bit i) /= 0 which rely on Num (and on the fact that 0 ~= 0..0 and 1 ~= 0..01, which doesn't have to be the case in all Num instances?). But is that worth having Num as superclass? When declaring in instance for Bits you have to implement at least 8 functions anyway so these two IMHO don't really make a difference, do they? //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
2009/5/8 Stephan Friedrichs deduktionstheo...@web.de: Hi! 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? We could declare instance Num Bool where (+) False = id (+) True = not (*) True True = True (*) _ _ = False (-) = (+) negate = id abs = id signum = const True fromInteger = not . even which basically implements the field with 2 elements and instance Bits Bool where bitSize = const 1 isSigned = const False (..) = () (.|.) = (||) xor = (+) complement = not shift = const shiftL = const shiftR = const rotate = const rotateL = const rotateR = const bit = (==0) setBit _ 0 = True setBit b _ = b clearBit _ 0 = False clearBit b _ = b complementBit b 0 = not b complementBit b _ = b testBit b 0 = b testBit _ _ = False quite trivial... Why is this not part of base? Or am I missing something? //Stephan Isn't XOR for booleans (/=)? Deniz Dogan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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 === Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html === ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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.sittampa...@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 === Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html === ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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 ndmitch...@gmail.com 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
Nope, it's in the Haskell standard. It means we can type: 1 + (2 :: Int) and have it work Otherwise what type would 1 have? Integer? Float? It's just a way of giving constants the type :: Num a = a On Fri, May 8, 2009 at 4:53 PM, Andrew Wagner wagner.and...@gmail.com wrote: 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 ndmitch...@gmail.com 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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. Of course, Lennart has been asking for precisely this functionality (overloaded Booleans) for some time - so one day it may be possible! You could however do: if 3 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... Yeah, the more people give examples of the power of Num Bool, the more it seems like a very bad idea! Which is a shame, having Bits on Bool seems entirely logical, having Num a superclass of Bits seems a little less clear. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
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] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
Am Freitag 08 Mai 2009 19:18:37 schrieb Brandon S. Allbery KF8NH: 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 fromInteger _ = unsafePerformIO $ randomRIO (False,True) looks safer to me. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
