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List: haskell-cafe
Subject: Re: [Haskell-cafe] Haskell Report Infelicity?
From: Dominic Steinitz <dominic () steinitz ! org>
Date: 2015-10-27 9:38:01
Message-ID: 254E618C-39FE-4B14-8636-525B279EABBC () steinitz ! org
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Hi Levent,
Thanks for this although it made me feel sad that this has not been resolved in 7 \
years. I just had a quick look at what OCAML does and they define floor :: Double -> \
Double (the don't really but that's the gist of it). Python, in so far as it is \
typed, does the same. So NaN can just be passed straight through for the user to deal \
with (no performance hit).
Dominic Steinitz
dominic@steinitz.org
http://idontgetoutmuch.wordpress.com
> On 26 Oct 2015, at 20:54, Levent Erkok <erkokl@gmail.com> wrote:
>
> There's an existing ticket about this, filed 7 years ago: \
> https://ghc.haskell.org/trac/ghc/ticket/3070 \
> <https://ghc.haskell.org/trac/ghc/ticket/3070>
> On Mon, Oct 26, 2015 at 1:49 PM, Dominic Steinitz <dominic@steinitz.org \
> <mailto:dominic@steinitz.org>> wrote: I have probably misunderstood but is the \
> following expected? floor, round, truncate and ceiling give similar results (all \
> methods of the RealFrac class).
> properFraction :: (Integral b, RealFrac a) => a -> (b, a)
>
> *Girsanov> properFraction (-1 / 0)
> (-17976931348623159077293051907890247336179769789423065727343008115773267
> 5805500963132708477322407536021120113879871393357658789768814416622492847
> 4306394741243777678934248654852763022196012460941194530829520850057688381
> 5068234246288147391311054082723716335051068458629823994724593847971630483
> 5356329624224137216,0.0)
> *Girsanov> properFraction (1 / 0)
> (179769313486231590772930519078902473361797697894230657273430081157732675
> 8055009631327084773224075360211201138798713933576587897688144166224928474
> 3063947412437776789342486548527630221960124609411945308295208500576883815
> 0682342462881473913110540827237163350510684586298239947245938479716304835
> 356329624224137216,0.0)
> *Girsanov> properFraction (0 / 0)
> (-26965397022934738615939577861835371004269654684134598591014512173659901
> 3708251444699062715983611304031680170819807090036488184653221624933739271
> 1459592111865666518401372982279144533294018691411791796244281275086532572
> 2602351369432221086966581124085574502576602687944735992086890771957445725
> 3034494436336205824,0.0)
>
> The Haskell Report "6.4.6 Coercions and Component Extraction" is
> silent on what to expect with NaN, Infinity and -Infinity so I think it should
> be amended. Alternatively, one could expect implementations to throw a
> runtime error?
>
> In ghc, the problem is here https://phabricator.haskell.org/D160a \
> <https://phabricator.haskell.org/D160a>
> /* This is expected to replace uses of __decodeDouble_2Int() in the long run */
> StgInt
> __decodeDouble_Int64 (StgInt64 *const mantissa, const StgDouble dbl)
> {
> if (dbl) {
> int exp = 0;
> *mantissa = (StgInt64)scalbn(frexp(dbl, &exp), DBL_MANT_DIG);
> return exp-DBL_MANT_DIG;
> } else {
> *mantissa = 0;
> return 0;
> }
> }
>
> I believe frexp [1] and scalbn [2] both pass through NaN, Infinity and -Infinity \
> and I think type coercion in C is undefined for these values.
>
> Furthermore exp is undefined for these values [1].
>
> 1. http://en.cppreference.com/w/cpp/numeric/math/frexp \
> <http://en.cppreference.com/w/cpp/numeric/math/frexp> 2. \
> http://en.cppreference.com/w/cpp/numeric/math/scalbn \
> <http://en.cppreference.com/w/cpp/numeric/math/scalbn>
> Dominic Steinitz
> dominic@steinitz.org <mailto:dominic@steinitz.org>
> http://idontgetoutmuch.wordpress.com <http://idontgetoutmuch.wordpress.com/>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org <mailto:Haskell-Cafe@haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe \
> <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>
[Attachment #5 (unknown)]
<html><head><meta http-equiv="Content-Type" content="text/html \
charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; \
-webkit-line-break: after-white-space;" class="">Hi Levent,<div class=""><br \
class=""></div><div class="">Thanks for this although it made me feel sad that this \
has not been resolved in 7 years. I just had a quick look at what OCAML does and they \
define floor :: Double -> Double (the don't really but that's the gist of it). \
Python, in so far as it is typed, does the same. So NaN can just be passed straight \
through for the user to deal with (no performance hit).</div><div class=""><br \
class=""><div apple-content-edited="true" class=""> <div class="">Dominic \
Steinitz</div><div class=""><a href="mailto:dominic@steinitz.org" \
class="">dominic@steinitz.org</a></div><div class=""><a \
href="http://idontgetoutmuch.wordpress.com" \
class="">http://idontgetoutmuch.wordpress.com</a></div>
</div>
<br class=""><div><blockquote type="cite" class=""><div class="">On 26 Oct 2015, at \
20:54, Levent Erkok <<a href="mailto:erkokl@gmail.com" \
class="">erkokl@gmail.com</a>> wrote:</div><br \
class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div \
class="">There's an existing ticket about this, filed 7 years ago: <a \
href="https://ghc.haskell.org/trac/ghc/ticket/3070" \
class="">https://ghc.haskell.org/trac/ghc/ticket/3070</a></div></div><div \
class="gmail_extra"><br class=""><div class="gmail_quote">On Mon, Oct 26, 2015 at \
1:49 PM, Dominic Steinitz <span dir="ltr" class=""><<a \
href="mailto:dominic@steinitz.org" target="_blank" \
class="">dominic@steinitz.org</a>></span> wrote:<br class=""><blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex">I have probably misunderstood but is the following expected? \
floor, round, truncate and ceiling give similar results (all methods of the RealFrac \
class).<br class=""> <br class="">
properFraction :: (Integral b, RealFrac a) => a -> (b, a)<br class="">
<br class="">
*Girsanov> properFraction (-1 / 0)<br class="">
(-17976931348623159077293051907890247336179769789423065727343008115773267<br \
class=""> 5805500963132708477322407536021120113879871393357658789768814416622492847<br \
class=""> 4306394741243777678934248654852763022196012460941194530829520850057688381<br \
class=""> 5068234246288147391311054082723716335051068458629823994724593847971630483<br \
class=""> 5356329624224137216,0.0)<br class="">
*Girsanov> properFraction (1 / 0)<br class="">
(179769313486231590772930519078902473361797697894230657273430081157732675<br \
class=""> 8055009631327084773224075360211201138798713933576587897688144166224928474<br \
class=""> 3063947412437776789342486548527630221960124609411945308295208500576883815<br \
class=""> 0682342462881473913110540827237163350510684586298239947245938479716304835<br \
class=""> 356329624224137216,0.0)<br class="">
*Girsanov> properFraction (0 / 0)<br class="">
(-26965397022934738615939577861835371004269654684134598591014512173659901<br \
class=""> 3708251444699062715983611304031680170819807090036488184653221624933739271<br \
class=""> 1459592111865666518401372982279144533294018691411791796244281275086532572<br \
class=""> 2602351369432221086966581124085574502576602687944735992086890771957445725<br \
class=""> 3034494436336205824,0.0)<br class="">
<br class="">
The Haskell Report "6.4.6 Coercions and Component Extraction" is<br class="">
silent on what to expect with NaN, Infinity and -Infinity so I think it should<br \
class=""> be amended. Alternatively, one could expect implementations to throw a<br \
class=""> runtime error?<br class="">
<br class="">
In ghc, the problem is here <a href="https://phabricator.haskell.org/D160a" \
rel="noreferrer" target="_blank" \
class="">https://phabricator.haskell.org/D160a</a><br class=""> <br class="">
/* This is expected to replace uses of __decodeDouble_2Int() in the long run */<br \
class=""> StgInt<br class="">
__decodeDouble_Int64 (StgInt64 *const mantissa, const StgDouble dbl)<br class="">
{<br class="">
if (dbl) {<br class="">
int exp = 0;<br class="">
*mantissa = (StgInt64)scalbn(frexp(dbl, &exp), \
DBL_MANT_DIG);<br class=""> return exp-DBL_MANT_DIG;<br \
class=""> } else {<br class="">
*mantissa = 0;<br class="">
return 0;<br class="">
}<br class="">
}<br class="">
<br class="">
I believe frexp [1] and scalbn [2] both pass through NaN, Infinity and -Infinity and \
I<br class=""> think type coercion in C is undefined for these values.<br class="">
<br class="">
Furthermore exp is undefined for these values [1].<br class="">
<br class="">
1. <a href="http://en.cppreference.com/w/cpp/numeric/math/frexp" rel="noreferrer" \
target="_blank" class="">http://en.cppreference.com/w/cpp/numeric/math/frexp</a><br \
class=""> 2. <a href="http://en.cppreference.com/w/cpp/numeric/math/scalbn" \
rel="noreferrer" target="_blank" \
class="">http://en.cppreference.com/w/cpp/numeric/math/scalbn</a><br class=""> <br \
class=""> Dominic Steinitz<br class="">
<a href="mailto:dominic@steinitz.org" class="">dominic@steinitz.org</a><br class="">
<a href="http://idontgetoutmuch.wordpress.com/" rel="noreferrer" target="_blank" \
class="">http://idontgetoutmuch.wordpress.com</a><br class=""> <br class="">
_______________________________________________<br class="">
Haskell-Cafe mailing list<br class="">
<a href="mailto:Haskell-Cafe@haskell.org" class="">Haskell-Cafe@haskell.org</a><br \
class=""> <a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe" \
rel="noreferrer" target="_blank" \
class="">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe</a><br \
class=""> </blockquote></div><br class=""></div>
</div></blockquote></div><br class=""></div></body></html>
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