Re: [Haskell-cafe] Re: Difference between div and /
Richard O'Keefe schrieb: On Jun 3, 2010, at 1:13 AM, Maciej Piechotka wrote: On Wed, 2010-06-02 at 14:01 +1200, Richard O'Keefe wrote: For what applications is it useful to use the same symbol for operations obeying (or in the case of floating point operations, *approximating* operations obeying) distinct laws? If the given operations do share something in common. For example * is usually commutative. However you do use it with quaternions (Hamilton product). You even write ij = k despite the fact that ji = -k. I think you just made my point: Commutativity is NOT one of the standard properties that * is EXPECTED to possess. However, it IS one of the properties that + is expected to possess, which is why Java's abuse of + for string concatenation is so bad. Java's (+) is not even associative: (text + 2) + 3 = text23 text + (2+3) = text5 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Difference between div and /
On Thu, 2010-06-03 at 12:44 +1200, Richard O'Keefe wrote: On Jun 3, 2010, at 1:13 AM, Maciej Piechotka wrote: On Wed, 2010-06-02 at 14:01 +1200, Richard O'Keefe wrote: For what applications is it useful to use the same symbol for operations obeying (or in the case of floating point operations, *approximating* operations obeying) distinct laws? If the given operations do share something in common. For example * is usually commutative. However you do use it with quaternions (Hamilton product). You even write ij = k despite the fact that ji = -k. I think you just made my point: Commutativity is NOT one of the standard properties that * is EXPECTED to possess. I don't think that many people expect * to be not commutative (I'm not speaking about people who deal with Mathematics - I mean 'average person' and 'average programmer'). If you look at the Int and Double instance of Random in the Random.hs that comes with Hugs, you'll see they use different code. It's not because of any problem with / per se but because they need genuinely different algorithms. Point taken. Regards signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Difference between div and /
On Wed, 2010-06-02 at 14:01 +1200, Richard O'Keefe wrote: For what applications is it useful to use the same symbol for operations obeying (or in the case of floating point operations, *approximating* operations obeying) distinct laws? If the given operations do share something in common. For example * is usually commutative. However you do use it with quaternions (Hamilton product). You even write ij = k despite the fact that ji = -k. I gave the code which might have work for both Integral and Fractional but it is not possible to type it in Haskell. Although I wouldn't mind something like: class Num a = Divisable a where (./.) :: a - a - a class (Real a, Enum a, Divisable a) = Integral a where div = (./.) ... class Divisable a = Fractional a where (/) = (./.) ... (/ and div preserve their meaning, ./. is the generalized division) Regards signature.asc 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] Re: Difference between div and /
On Wed, 2 Jun 2010, Maciej Piechotka wrote: On Wed, 2010-06-02 at 14:01 +1200, Richard O'Keefe wrote: For what applications is it useful to use the same symbol for operations obeying (or in the case of floating point operations, *approximating* operations obeying) distinct laws? If the given operations do share something in common. For example * is usually commutative. However you do use it with quaternions (Hamilton product). You even write ij = k despite the fact that ji = -k. I do not like to see the type class mechanism as a way to use common identifiers and symbols in as many as possible applications. Instead for me type classes are a way to write algorithms in a way that they can be used for many particular types. So far I had no algorithm that works equally well on integral 'div' and fractional '/'. Can you give me an example of an algorithm, where in one case instantiation to Integer and 'div' is sensible and in another case instantation to Rational and '/' is sensible? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Difference between div and /
Sorry, I missed this post. Maciej Piechotka schrieb: Well - i tried to write some package dealing with distributions etc. If you have something like that: instance ... = Distribution (Linear a) a where rand (Linear f s) g = let (gf, gt) = genRange g (v, g') = next g in (g', f + (fromIntegral v * s) / fromIntegral (gt - gf)) (I haven't check it but IMHO it is right implementation) Now I have following options: - Implement per Int/Int8/... - Implement IntegerLinear and FractionalLinear separatly That is, what you need is a general division with rounding. But you might more generally want a custom type class with a method that selects an element from a set for given parameters gf, gt, v. This way, you could also handle distributions on Enumeration types. You certainly you do not want, say a division operation on Monday, Tuesday, ..., Sunday, but having a probability distribution of weekdays is very reasonable. Btw. you may want to have a look at: http://hackage.haskell.org/package/probability ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Difference between div and /
On Jun 3, 2010, at 1:13 AM, Maciej Piechotka wrote: On Wed, 2010-06-02 at 14:01 +1200, Richard O'Keefe wrote: For what applications is it useful to use the same symbol for operations obeying (or in the case of floating point operations, *approximating* operations obeying) distinct laws? If the given operations do share something in common. For example * is usually commutative. However you do use it with quaternions (Hamilton product). You even write ij = k despite the fact that ji = -k. I think you just made my point: Commutativity is NOT one of the standard properties that * is EXPECTED to possess. However, it IS one of the properties that + is expected to possess, which is why Java's abuse of + for string concatenation is so bad. I gave the code which might have work for both Integral and Fractional but it is not possible to type it in Haskell. This is what you wrote: If you have something like that: instance ... = Distribution (Linear a) a where rand (Linear f s) g = let (gf, gt) = genRange g (v, g') = next g in (g', f + (fromIntegral v * s) / fromIntegral (gt - gf)) Since I don't know what Linear is or what Distribution is it's hard for me to tell exactly what this is supposed to do, but I'll take a stab at it. (Linear f s) specifies an interval by giving a starting point f and a width s; the interval specified is the half-open interval [f,f+s). genRange g returns (l,u) where l is the smallest Int that g can return and u is the largest Int that g can return; these are INCLUSIVE bounds so the number of values to consider is u-l+1. For StdGen the bounds are (0,2147483562) Let's just see what would happen if - we used this code - g was an instance of StdGen - f was 0 :: Int - s was 100 :: Int next g -- (1079700,g') -- actual run fromIntegral 1079700 * 100 = 10797 = 1663208704 :: Int 1663208704 `div` fromIntegral (2147483562 - 0) :: Int == 0 In fact almost all the answers you get will be 0. If f and s are of any bounded integral type, this is NOT going to do anything useful. And it's NOT because / is not aliased to div. It's because multiplication overflows. If you look at the Int and Double instance of Random in the Random.hs that comes with Hugs, you'll see they use different code. It's not because of any problem with / per se but because they need genuinely different algorithms. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Difference between div and /
On Tue, 2010-06-01 at 15:29 -0700, Evan Laforge wrote: [1] By co I mean Ruby, Python, Perl and others. There are no so many languages that do recognize the difference. % python -Q new Python 2.4.6 (#1, Aug 3 2009, 17:05:16) [GCC 4.0.1 (Apple Inc. build 5490)] on darwin Type help, copyright, credits or license for more information. 10 / 3 #- 3.3335 10 // 3 #- 3 The python guys decided that int/int - int was a mistake, but because it's an incompatible change, the removal process has been long (hence the -Q flag, or a from __future__ import). In fact, I think they gave up on making it the default before python 3. I appreciate that haskell has differentiated from the beginning. Well - i tried to write some package dealing with distributions etc. If you have something like that: instance ... = Distribution (Linear a) a where rand (Linear f s) g = let (gf, gt) = genRange g (v, g') = next g in (g', f + (fromIntegral v * s) / fromIntegral (gt - gf)) (I haven't check it but IMHO it is right implementation) Now I have following options: - Implement per Int/Int8/... - Implement IntegerLinear and FractionalLinear separatly Neither of choices are IMHO not ideal. Regards signature.asc 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] Re: Difference between div and /
On Wednesday 02 June 2010 00:55:08, Maciej Piechotka wrote: On Tue, 2010-06-01 at 15:29 -0700, Evan Laforge wrote: [1] By co I mean Ruby, Python, Perl and others. There are no so many languages that do recognize the difference. % python -Q new Python 2.4.6 (#1, Aug 3 2009, 17:05:16) [GCC 4.0.1 (Apple Inc. build 5490)] on darwin Type help, copyright, credits or license for more information. 10 / 3 #- 3.3335 10 // 3 #- 3 The python guys decided that int/int - int was a mistake, but because it's an incompatible change, the removal process has been long (hence the -Q flag, or a from __future__ import). In fact, I think they gave up on making it the default before python 3. I appreciate that haskell has differentiated from the beginning. Well - i tried to write some package dealing with distributions etc. If you have something like that: instance ... = Distribution (Linear a) a where rand (Linear f s) g = let (gf, gt) = genRange g (v, g') = next g in (g', f + (fromIntegral v * s) / fromIntegral (gt - gf)) (I haven't check it but IMHO it is right implementation) Now I have following options: - Implement per Int/Int8/... - Implement IntegerLinear and FractionalLinear separatly - use realToFrac instead of fromIntegral (using the logfloat package is probably not a bad idea then) Neither of choices are IMHO not ideal. Methinks that is not what you wanted to say ;) Regards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Difference between div and /
On Wed, 2010-06-02 at 01:13 +0200, Daniel Fischer wrote: On Wednesday 02 June 2010 00:55:08, Maciej Piechotka wrote: On Tue, 2010-06-01 at 15:29 -0700, Evan Laforge wrote: [1] By co I mean Ruby, Python, Perl and others. There are no so many languages that do recognize the difference. % python -Q new Python 2.4.6 (#1, Aug 3 2009, 17:05:16) [GCC 4.0.1 (Apple Inc. build 5490)] on darwin Type help, copyright, credits or license for more information. 10 / 3 #- 3.3335 10 // 3 #- 3 The python guys decided that int/int - int was a mistake, but because it's an incompatible change, the removal process has been long (hence the -Q flag, or a from __future__ import). In fact, I think they gave up on making it the default before python 3. I appreciate that haskell has differentiated from the beginning. Well - i tried to write some package dealing with distributions etc. If you have something like that: instance ... = Distribution (Linear a) a where rand (Linear f s) g = let (gf, gt) = genRange g (v, g') = next g in (g', f + (fromIntegral v * s) / fromIntegral (gt - gf)) (I haven't check it but IMHO it is right implementation) Now I have following options: - Implement per Int/Int8/... - Implement IntegerLinear and FractionalLinear separatly - use realToFrac instead of fromIntegral (using the logfloat package is probably not a bad idea then) I'm not quire sure how to use it. I would have to either use floor/... which would make result Integral or left it as it is and having (Fractional a, Real a) constraint. Neither of choices are IMHO not ideal. Methinks that is not what you wanted to say ;) Ups. Sorry - it's rather late and I'm not native speaker (and my native language do use double negation). Neither of the choices are ideal IMHO. Regards Regards signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
