RE: Double -> CDouble, realToFrac doesn't work
| Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm? I don't recall a discussion about this, and a quick search in my mail archive didn't turn up anything except the enclosed from George Russell. I'm pretty ignorant about the dark corners of numerics, but it does seem bad that passing through Rational loses information. Perhaps the Report should specify normalised representations for +Inf, -Inf and NaN as Rationals (1:%0, -1:%0, and 0:%0 seem like plausible candidates). If someone wants to try this out, and send us a patch for the Rational library, we could incorporate it. And so far as the report goes, perhaps the Errata could contain a note identifying the issue, and suggesting a solution. It's a bit late to *specify* a solution unless we are really sure about it. Simon -----Original Message----- From: George Russell [mailto:ger@informatik.uni-bremen.de] Sent: 25 February 2000 10:19 To: glasgow-haskell-users@haskell.org Subject: Floating-point nitpicking: floor(Inf) and floor(NaN) floor(Inf) and floor(NaN) do not appear to be defined in Standard Haskell. (They both come down to "properFraction" which is only defined for Ratio.) This differs from (for example) the Standard ML Basis Library, where it is specified that floor(Int) should raise Overflow and floor(NaN) should raise Domain. Hence Hugs and GHC do different things. Hugs returns floor(Inf) = 0 and floor(NaN) = 0 GHC returns floor(Inf) = very very large integer and floor(NaN) = even larger integer. (This is because the GHC implementation of properFraction simply ignores the case of Inf/NaN and treats the artificially high exponent encoded in those floating-point numbers as if it were a real one.) My own opinion is that Standard ML is right here and that floor(x) should raise an exception (In Haskell terms, fail) when x does not correspond to a real number. | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Sven Panne | Sent: 08 August 2004 17:02 | To: Hal Daume III | Cc: GHC Users Mailing List; Malcom Wallace | Subject: Re: Double -> CDouble, realToFrac doesn't work | | Hal Daume III wrote: | > [...] | > Prelude Foreign.C> (0 :: CDouble) / 0 | > NaN | > Prelude Foreign.C> (0 :: Double) / 0 | > NaN | > Prelude Foreign.C> realToFrac ((0 :: Double) / 0) :: CDouble | > -Infinity | > | > yikes! the NaN got turned into a -Infinity!!! | > | > aside from manually checking for 'strange' Double/CDouble values and | > wrapping realToFrac, is there a better way? also, does this count as a | > bug? | | Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm? | | When you compile the stuff above with optimizations on, you get what you've | expected, thanks to RULES which shortcut the route via Rational completely. | | Cheers, | S. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
It's an old thread, but nothing has really happened yet, so I'd like to restate and expand the question: What should the behaviour of toRational, fromRational, and decodeFloat for NaN and +/-Infinity be? Even if the report is unclear here, it would be nice if GHC, Hugs, and NHC98 agreed on something. Can we agree on the special Rational values below? Printing those values could be more consistent for these systems, too, BTW. Cheers, S. Simon Peyton-Jones wrote:
| Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm?
I don't recall a discussion about this, and a quick search in my mail archive didn't turn up anything except the enclosed from George Russell.
I'm pretty ignorant about the dark corners of numerics, but it does seem bad that passing through Rational loses information. Perhaps the Report should specify normalised representations for +Inf, -Inf and NaN as Rationals (1:%0, -1:%0, and 0:%0 seem like plausible candidates).
If someone wants to try this out, and send us a patch for the Rational library, we could incorporate it. And so far as the report goes, perhaps the Errata could contain a note identifying the issue, and suggesting a solution. It's a bit late to *specify* a solution unless we are really sure about it.
Simon
-----Original Message----- From: George Russell [mailto:ger@informatik.uni-bremen.de] Sent: 25 February 2000 10:19 To: glasgow-haskell-users@haskell.org Subject: Floating-point nitpicking: floor(Inf) and floor(NaN)
floor(Inf) and floor(NaN) do not appear to be defined in Standard Haskell. (They both come down to "properFraction" which is only defined for Ratio.) This differs from (for example) the Standard ML Basis Library, where it is specified that floor(Int) should raise Overflow and floor(NaN) should raise Domain. Hence Hugs and GHC do different things.
Hugs returns floor(Inf) = 0 and floor(NaN) = 0 GHC returns floor(Inf) = very very large integer and floor(NaN) = even larger integer. (This is because the GHC implementation of properFraction simply ignores the case of Inf/NaN and treats the artificially high exponent encoded in those floating-point numbers as if it were a real one.)
My own opinion is that Standard ML is right here and that floor(x) should raise an exception (In Haskell terms, fail) when x does not correspond to a real number.
| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Sven Panne | Sent: 08 August 2004 17:02 | To: Hal Daume III | Cc: GHC Users Mailing List; Malcom Wallace | Subject: Re: Double -> CDouble, realToFrac doesn't work | | Hal Daume III wrote: | > [...] | > Prelude Foreign.C> (0 :: CDouble) / 0 | > NaN | > Prelude Foreign.C> (0 :: Double) / 0 | > NaN | > Prelude Foreign.C> realToFrac ((0 :: Double) / 0) :: CDouble | > -Infinity | > | > yikes! the NaN got turned into a -Infinity!!! | > | > aside from manually checking for 'strange' Double/CDouble values and | > wrapping realToFrac, is there a better way? also, does this count as a | > bug? | | Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm? | | When you compile the stuff above with optimizations on, you get what you've | expected, thanks to RULES which shortcut the route via Rational completely. | | Cheers, | S. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
On Thu, Nov 04, 2004 at 08:32:52PM +0100, Sven Panne wrote:
It's an old thread, but nothing has really happened yet, so I'd like to restate and expand the question: What should the behaviour of toRational, fromRational, and decodeFloat for NaN and +/-Infinity be? Even if the report is unclear here, it would be nice if GHC, Hugs, and NHC98 agreed on something. Can we agree on the special Rational values below?
I would be very careful of adding non-rationals to the Rational type. For one thing, it breaks the traditional rule for equality a % b == c % d iff a*d == b*c You'd need to look at all the instances for Ratio a that are defined. For instance, the Ord instance would require at least lots of special cases. And when would you expect 'x/0' to give +Infinity and when -Infinity? For IEEE floats, there are distinct representations of +0 and -0, which lets you know when you want which one. But for the Rational type there is no such distinction. The behaviour that '1 % 0' gives the error 'Ratio.% : zero denominator' is clearly specified by the Library Report. In the meantime, there are utility functions for dealing with IEEE floats (isNaN, etc.) Peace, Dylan
With GHCi, I get: Prelude Ratio> toRational (1.0/0) :: Ratio Integer 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216 % 1 My personal gut feeling is that converting a NaN/Inf/etc to a fraction should be an error (although I see the point of being able to convert a e.g. Float to a Double and preserve such special values). Is it possible to provide a conversion function in the Floating or RealFloat class instead? -kzm -- If I haven't seen further, it is by standing in the footprints of giants
participants (4)
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dpt＠lotus.bostoncoop.net -
Ketil Malde -
Simon Peyton-Jones -
Sven Panne