float & fraction equality bug

On 11/10/2017 4:18 PM, Nicolas Cellier wrote:

2017-11-10 20:58 GMT+01:00 Martin McClure <[hidden email]>:

On 11/10/2017 11:33 AM, [hidden email] wrote:

Doing only Fraction->Float conversions in mixed mode won't preserve = as
an equivalence relation and won't enable a consistent ordering with <=,
which probably most Smalltalkers consider important and enjoyable
properties.

Good point. I agree that Float -> Fraction is the more desirable mode for implicit conversion, since it can always be done without changing the value.

Nicolas gave some convincing examples on why most
programmers might want to rely on them.


Also, as I mentioned, most Smalltalkers might prefer keeping away from
the complex properties of Floats. Doing automatic, implicit
Fraction->Float conversions behind the scenes only exacerbates the
probability of encountering Floats and of having to deal with their
weird and unfamiliar arithmetic.

One problem is that we make it easy to create Floats in source code (0.1), and we print Floats in a nice decimal format but by default print Fractions in their reduced fractional form. If we didn't do this, Smalltalkers might not be working with Floats in the first place, and if they did not have any Floats in their computation they would never run into an implicit conversion to *or* from Float.

As it is, if we were to uniformly do Float -> Fraction conversion on mixed-mode operations, we would get things like

(0.1 * (1/1)) printString                       -->
'3602879701896397/36028797018963968'

Not incredibly friendly.

For those not practicing the litote: definitely a no go.

Regards,
-Martin

At the risk of repeating myself, unique choice for all operations is a nice to have but not a goal per se.

I mostly agree with Florin: having 0.1 representing a decimal rather than a Float might be a better path meeting more expectations.

But thinking that it will magically eradicate the problems is a myth.

Shall precision be limited or shall we use ScaledDecimals ?

With limited precision, we'll be back to having several Fraction converting to same LimitedDecimal, so it won't solve anything wrt original problem.

I don't think people have problems with understanding limited precision. We do know that when we write 3.14.. we did not write Pi.
As long as it does not contradict the intuition that we developed early on (and that's the problem with the binary representation of floats, not that it is limited)...

With illimted precision we'll have the bad property that what we print does not re-interpret to the same ScaledDecimal (0.1s / 0.3s), but that's a detail.

The worse thing is that long chain of operations will tend to produce monster numerators denominators.

And we will all have long chain when resizing a morph with proportional layout in scalable graphics.

But I did not propose to drop floats, just to change the visible representation of literals: 0.1 would mean a ScaledDecimal literal and 0.1f would mean a float literal. The extra "f" would be a constant reminder that something special is going on (the dangers of interpreting the literal as an exact representation in base 10).

We will then have to insert rounding operations manually for mitigating the problem, and somehow reinvent a more inconvenient Float...

I don't think we would have to reinvent Float - this one would stay exactly the same (other than printing the extra f (or d)).
But I agree that you raise an important point with precision/scale. We would indeed need to make ScaledDecimal more complicated.
Because today 0.1s * 0.1s is printed as 0.0s1, even though 0.1s * 0.1s = (1/100) evaluates to true, which is nice.

It's boring to allways play the role of Cassandra, but why do you think that Scheme and Lisp did not choose that path?

Oh. come on, if Scheme and Lisp did it all already, why are we all here? :)

On 11/10/2017 4:42 PM, Florin Mateoc wrote:

On 11/10/2017 4:18 PM, Nicolas Cellier wrote:

2017-11-10 20:58 GMT+01:00 Martin McClure <[hidden email]>:

On 11/10/2017 11:33 AM, [hidden email] wrote:

Doing only Fraction->Float conversions in mixed mode won't preserve = as
an equivalence relation and won't enable a consistent ordering with <=,
which probably most Smalltalkers consider important and enjoyable
properties.

Good point. I agree that Float -> Fraction is the more desirable mode for implicit conversion, since it can always be done without changing the value.

Nicolas gave some convincing examples on why most
programmers might want to rely on them.


Also, as I mentioned, most Smalltalkers might prefer keeping away from
the complex properties of Floats. Doing automatic, implicit
Fraction->Float conversions behind the scenes only exacerbates the
probability of encountering Floats and of having to deal with their
weird and unfamiliar arithmetic.

One problem is that we make it easy to create Floats in source code (0.1), and we print Floats in a nice decimal format but by default print Fractions in their reduced fractional form. If we didn't do this, Smalltalkers might not be working with Floats in the first place, and if they did not have any Floats in their computation they would never run into an implicit conversion to *or* from Float.

As it is, if we were to uniformly do Float -> Fraction conversion on mixed-mode operations, we would get things like

(0.1 * (1/1)) printString                       -->
'3602879701896397/36028797018963968'

Not incredibly friendly.

For those not practicing the litote: definitely a no go.

Regards,
-Martin

At the risk of repeating myself, unique choice for all operations is a nice to have but not a goal per se.

I mostly agree with Florin: having 0.1 representing a decimal rather than a Float might be a better path meeting more expectations.

But thinking that it will magically eradicate the problems is a myth.

Shall precision be limited or shall we use ScaledDecimals ?

With limited precision, we'll be back to having several Fraction converting to same LimitedDecimal, so it won't solve anything wrt original problem.

I don't think people have problems with understanding limited precision. We do know that when we write 3.14.. we did not write Pi.
As long as it does not contradict the intuition that we developed early on (and that's the problem with the binary representation of floats, not that it is limited)...

With illimted precision we'll have the bad property that what we print does not re-interpret to the same ScaledDecimal (0.1s / 0.3s), but that's a detail.

The worse thing is that long chain of operations will tend to produce monster numerators denominators.

And we will all have long chain when resizing a morph with proportional layout in scalable graphics.

But I did not propose to drop floats, just to change the visible representation of literals: 0.1 would mean a ScaledDecimal literal and 0.1f would mean a float literal. The extra "f" would be a constant reminder that something special is going on (the dangers of interpreting the literal as an exact representation in base 10).

And there would still be a lot of places where we would continue to judiciously use floats, such as in morphic. Just like today people don't use fractions when they don't need their extra precision - ah, who am I kidding?

There are a lot of developers out there who would go to great lengths to save an extra keystroke. :)
Well, I think it could be made to work (e.g. one of the low-hanging fixes would be that multiplication of scaledDecimals would add their scales) and it would complement nicely Smalltalk's seamless and intuitive largeIntegers - just like largeIntegers liberate us from the tyranny of the hardware representation for integers, we should also not be prisoners to the hardware representation of floats.