[Haskell-cafe] 1/0

[Don Stewart]

droundy:
> On Mon, Jun 16, 2008 at 05:08:36PM -0700, Don Stewart wrote:
> > droundy:
> > > On Mon, Jun 16, 2008 at 04:50:05PM -0700, John Meacham wrote:
> > > > On Mon, Jun 16, 2008 at 04:41:23PM -0700, Evan Laforge wrote:
> > > > > But what about that NaN->Integer conversion thing?
> > > > 
> > > > I think that may be a bug or at least a misfeature. The standard is
> > > > somewhat vauge on a lot of issues dealing with floating point since
> > > > it is such a tricky subject and depends a lot on the environment. The
> > > > various rounding funcitons are particularly ugly IMHO. I added varients
> > > > of them that preserved the floating point type and properly implemented
> > > > IEEE behavior for jhc.
> > > 
> > > I think the Data.Binary guys think it's a feature, since they rely in
> > > this behavior (well, they rely on the equivalently-foolish behavior of
> > > toRational).  I think it's a bug.
> > 
> > You mean:
> > 
> >     instance Binary Double where
> >         put d = put (decodeFloat d)
> >         get   = liftM2 encodeFloat get get
> > 
> > ?
> > 
> > if you've a portable Double decoding that works in GHC and Hugs, I'm
> > accepting patches.
> 
> I really don't understand why being portable is such an issue.  Is it
> really better to behave wrong on every platform rather than behaving
> wrong on a very small minority of platforms?

The Binary instances are required to be portable, as that's the part
of the definition of Binary's mandate: a portable binary encoding.
  
> Anyhow, I've not hacked on binary, because I've not used it, and have
> trouble seeing when I would use it.  So maybe I shouldn't have brought
> the subject up, except that this use of decodeFloat/encodeFloat is a
> particularly egregious misuse of floating point numbers.  

> decodeFloat
> really ought to be a partial function, and this should be a crashing
> bug, if the standard libraries were better-written.

It's a bug in the H98 report then:

    Section 6.4.6:

    "The function decodeFloat applied to a real floating-point number returns
    the significand expressed as an Integer and an appropriately scaled
    exponent (an Int). If decodeFloat x yields (m,n), then x is equal in value
    to mb^n, where b is the floating-point radix, and furthermore, either m
    and n are both zero or else b^d-1<=m<b^d, where d is the value of
    floatDigits x.  encodeFloat performs the inverse of this transformation.
    "

-- Don