[MLton-user] Real32.realRound is broken

[John Reppy]

You're right.  The difference between Real64 and Real32 is that the  
fractional
part is not visible with the default number of significant digits used  
by toString.

	- John

On Apr 24, 2008, at 9:48 AM, Matthew Fluet wrote:

> On Wed, 23 Apr 2008, John Reppy wrote:
>> The Real32.realRound function in mlton does not work.  Here is a  
>> simple test
>> program:
>>
>> 	 fun snap f = 0.001 * Real32.realRound(f * 1000.0);
>> 	 fun g x = print(concat["x = ", Real32.toString x, ", snap(x) = ",
>> 	 Real32.toString(snap x), "\n"]);
>> 	 g 45.0000038147;
>>
>> The Real64 version of this program produces the correct output
>>
>> 	x = 45.0000038147, snap(x) = 45
>>
>> but the Real32 version produces
>>
>> 	x = 45.0000038147, snap(x) = 45.0000038147
>>
>> This is using MLton 20070826 on Mac OS X 10.5.2/Intel.
>
> Isn't this simply a consequence of IEEE floating point values being  
> represented with radix 2, so the base 10 value 0.001 cannot be  
> represented exactly?
>
> The following program:
>
> structure Real = Real32
> fun r2s x = Real.toString x
> val s = 0.001
> val z = 45000.0
> val sz = s * z
> val () = print(concat["s = ", r2s s, ", z = ", r2s z, ", s * z = ",  
> r2s sz, "\n"])
>
> produces
>
> s = 0.0010000000475, z = 45000, s * z = 45.0000038147
>
> so, if there is a bug, it isn't with Real32.realRound.