[Haskell-cafe] New OpenGL package: efficient way to convert
wren ng thornton
wren at freegeek.org
Wed Sep 30 23:33:59 EDT 2009
Peter Verswyvelen wrote:
> I don't want to use the GL types directly since the OpenGL renderer is not
> exposes in the rest of the API.
> I was hoping that realToFrac would be a nop in case it would be identical to
> an unsafeCoerce.
This is one of the areas where the H98 spec is broken. Not only is the
H98 realToFrac slow, it is also incorrect for Double and Float since
those types contain transfinite values which cannot be represented in
Rational, but which can be converted successfully when mapping from
Double to Float and vice versa.
Once you find a suitable function definition, I suggest using the
RealToFrac class. I don't know the GL types to know what the
definition should be, but there is a class already out there to help
deal with this kind of problem both efficiently and correctly.
> I guess one could make rules for that, but this tickets makes me wander if
> that really works:
The RealToFrac class solves Henning's performance complaint as well.
Because of the correctness issues involved here, I strongly advocate
*against* the use of rewrite rules for trying to optimize the H98
definition of realToFrac. The semantics of the program where transfinite
values are involved should not be contingent on the whims of how the
compiler feels today. Even though IEEE-754 is fuzzy to begin with,
altering whether things fall into or out of transfinite states is
And in case you're worried about "extraneous dependencies", the logfloat
package is portable and has negligable dependencies. Moreover it also
provides a number of other facilities for dealing with IEEE-754 numbers
correctly. Some examples include:
* correcting the definitions of isInfinite and isNaN used in Hugs,
* defining a class for partial orders (since neither Double nor Float is
actually a total order due to NaN),
* and a class for types with explicit representations of transfinite
values, which in turn allows for a modified definition of log which has
log 0 == negativeInfinity (other Floating methods could be extended
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