Re: problem(?) using java.lang.Float in arithmetic operation

[Per Bothner <per at bothner dot com>]

cr blimp wrote:
> One question - 
> I'm extracting a single float from a java float[]
> array returned
> from said library.  It appears that when such a float
> is passed to "-", i.e.:
>
>    (set! arr (java-library-returns-float))
>    (set! x (arr 0))
>    (set! y (- x))
>
> there is a type error -  
> procedure main throws exception
> java.lang.ClassCastException: java.lang.Float
> Invalid parameter, was: java.lang.Float

Looks like a semi-bug in gnu.kawa.functions.AddOp.
You'll note that apply2 (binary minues) has special
code to handle FLOAT_CODE.  However, the unary
version $MN(Object) casts the object to Numeric, without
trying to handle the other possibilities.  Hence,
the ClassCastException.

So one work-around might be to do:
  (set! y (- 0 x))

> Is there a function to convert a java.lang.Float to a
> scheme real?

You could probably apply the doubleValue method:

  (let ((x :: <java.lang.Float> arr 0))
    (- (x:doubleValue x)))

(This is untested.)

> If not, I could redefine all the basic arithmetic
> operations
> as calls to java.  Is this the appropriate approach?

The best solution is to extend $Mn(Object) to match the
switch in apply2(int plusOrMinus, Object arg1, Object arg2).
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/