Always false float comparisons

[Joe Duarte via Digitalmars-d]

On Monday, 9 May 2016 at 09:10:19 UTC, Walter Bright wrote:
> Don Clugston pointed out in his DConf 2016 talk that:
>
>     float f = 1.30;
>     assert(f == 1.30);
>
> will always be false since 1.30 is not representable as a 
> float. However,
>
>     float f = 1.30;
>     assert(f == cast(float)1.30);
>
> will be true.
>
> So, should the compiler emit a warning for the former case?

I think it really depends on what the warning actually says. I 
think people have different expectations for what that warning 
would be.

When you say 1.30 is not representable as a float, when is the 
"not representable" enforced? Because it looks like the 
programmer just represented it in the assignment of the literal – 
but that's not where the warning would be right? I mean I assume 
so because people need nonrational literals all the time, and 
this is the only way they can do it, which means it's a hole in 
the type system right? There should be a decimal type to cover 
all these cases, like some databases have.

Would the warning say that you can't compare 1.30 to a float 
because 1.30 is not representable as a float? Or would it say 
that f was rounded upon assignment and is no longer 1.30?

Short of a decimal type, I think it would be nice to have a 
"float equality" operator that covered this whole class of cases, 
where floats that started their lives as nonrational literals and 
floats that have been rounded with loss of precision can be 
treated as equal if they're within something like .0000001% of 
each other (well a  percentage that can actually be represented 
as a float...) Basically equality that covers the known 
mutational properties of fp arithmetic.

There's no way to do this right now without ranges right? I know 
that ~ is for concat. I saw ~= is an operator. What does that do? 
The Unicode ≈ would be nice for this.

I assume IEEE 754 or ISO 10967 don't cover this? I was just 
reading the latter (zip here: 
http://standards.iso.org/ittf/PubliclyAvailableStandards/c051317_ISO_IEC_10967-1_2012.zip)