[Haskell] Platform-dependent behaviour with functions on NaN

[Robby Findler]

I don't know if it would help, but PLT Scheme has been thru this and
Matthew Flatt has a nice test suite that you can see here:

  http://svn.plt-scheme.org/plt/trunk/collects/tests/mzscheme/number.ss

To help read the code, when you see something like:

  (test a b c ...)

that is the same thing as applying the operator "b" to the arguments "c
..." and then comparing the result to "a" (and expecting it to match).
A variation (defined in this file) is test-nan.0 that takes the "b" and
"c"s above, and passes not-a-number along as the expected result.

Sure enough, the bug below would have been caught by the test suite,
because it contains:

  (test-nan.0 ceiling +nan.0)

One could probably automate a translation of the test suite to
Quickcheck with some Emacs macros or (god forbid!) a Scheme program. :)

Anyways, if you do want to use those test cases, I'm happy to explain
any Schemeisms you find in there.

Just a Schemer's $0.02, 
Robby

At Fri, 14 Apr 2006 10:04:51 -0400, Lennart Augustsson wrote:
> Yeah, I think it boils down to different representations of NaN on
> different platform.  I guess I forgot to test for NaN when I wrote
> (the C code for) decodeFloat.  It should generate some consistent
> result.
> On the other hand, if you have code that possible divides by 0
> and don't check for it, well...
> It would be nice if Haskell allowed you to turn on signalling NaNs.
> 
> 	-- Lennart
> 
> Geisler, Tim (EXT) wrote:
> > In Haskell, the behaviour of functions on floating-point values which 
> > are NaN can be platform dependent.
> >  
> > On "SunOS sun 5.9 Generic_118558-09 sun4u sparc SUNW,Sun-Blade-1500":
> > Prelude> ceiling (0/0)
> > 
> 359538626972463141629054847463408713596141135051689993197834953606314521560057077521179117265
> 533756343080917907028764928468642653778928365536935093407075033972099821153102564152490980180
> 778657888151737016910267884609166473806445896331617118664246696549595652408289446337476354361
> 838599762500808052368249716736
> >  
> > On Windows:
> >  
> > Prelude> ceiling (0/0)
> > -
> 269653970229347386159395778618353710042696546841345985910145121736599013708251444699062715983
> 611304031680170819807090036488184653221624933739271145959211186566651840137298227914453329401
> 869141179179624428127508653257226023513694322210869665811240855745025766026879447359920868907
> 719574457253034494436336205824
> >  
> > Both machines use the binary distributions of GHC 6.4.1.
> >  
> > In our production code, this error (which is actually an error in our 
> > program) occured inside a quite complex expression which can be 
> > simplified to max 0 (ceiling (0/0)).  On Windows, we did not recognize 
> > the error in the program, because the complex expression just returned 
> > 0.  On Solaris, the complex expression returned this large number (which 
> > represents in the application "the number of CPUs needed in a certain 
> > device" ;-)
> >  
> > We develop Haskell programs on Windows and run them in production on 
> > Sparc with Solaris. It seems that we have to run special regression 
> > tests testing for differences between Sparc Solaris and Windows.
> >  
> > The Haskell 98 
> > report http://www.haskell.org/onlinereport/basic.html#sect6.4 states: 
> > "The results of exceptional conditions (such as overflow or underflow) 
> > on the fixed-precision numeric types are undefined; an implementation 
> > may choose error (/_|_/, semantically), a truncated value, or a special 
> > value such as infinity, indefinite, etc."
> >  
> > There has been some discussion six years ago and nearly two years ago: 
> > http://blog.gmane.org/gmane.comp.lang.haskell.glasgow.user/month=20040801
> >  
> > Is there a chance to
> > - properly define the behaviour of functions depending on the function 
> > properFraction for values like NaN, Infinity, ...?
> > - get an implementation of this in GHC which computes the same results 
> > for all platforms?
> >  
> > Perhaps the function properFraction could raise an exception in case of 
> > isNaN and isInfinity?
> >  
> > Other languages are more portable. E.g., for Java, these cases are 
> > defined. 
> > http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html#9249 states: 
> > "All numeric operations with NaN as an operand produce NaN as a result."
> >  
> > Tim
> > 
> > 
> > ------------------------------------------------------------------------
> > 
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