enable Float to percentage conversions

[Murmele]

Description: This allows to do calculations on relative values by using Floats and converting then to percentage by *1% or by dividing by 100. Example:

private property <float> f: 50;
private property <percent> p: (f / 2) * 1%;

private property <float> f: 50;
private property <percent> p: (f / 2) / 100;

Fixes: #3785

[ogoffart]

We need to discuss this when I'm back from vacation. I'm a bit worried that the implicit conversion may cause problem for cases where the behaviour is different depending on Whether the type is a % or not (eg, colour functions or width/height.)

[Murmele]

We need to discuss this when I'm back from vacation. I'm a bit worried that the implicit conversion may cause problem for cases where the behaviour is different depending on Whether the type is a % or not (eg, colour functions or width/height.)

Because of that I assigned to you. It is no rush to merge it :)
For length properties a unit must be specified additionally to the float value because there is no direct conversion from float to length

[ogoffart]

Another reason i'm not in favor of this is that code like that:

private property <percent> p: 50;

It looks like it is 50%, but it is instead 5000%

This means that forgetting an unit no longer result in a compilation error but now gives the wrong result, which is against the principle of type safety.

Ideally we should have the following to work:

private property <float> f: 50;
private property <percent> p: (f / 2) * 1%;

But it doesn't, because the multiplication by 1% does't change the type and maybe it should.

In other word, we should instead change the type of an expression that multiplies by a percentage, rather than having an automatic conversion.

[Murmele]

Another reason i'm not in favor of this is that code like that:

private property <percent> p: 50;

It looks like it is 50%, but it is instead 5000%

This means that forgetting an unit no longer result in a compilation error but now gives the wrong result, which is against the principle of type safety.

Ideally we should have the following to work:

private property <float> f: 50;
private property <percent> p: (f / 2) * 1%;

But it doesn't, because the multiplication by 1% does't change the type and maybe it should.

In other word, we should instead change the type of an expression that multiplies by a percentage, rather than having an automatic conversion.

This is a good point, I will create later some tests regarding those cases so we don't forget them. Do you think it would make sense to create an explicit function for it? Like float_to_percent() instead of multiplying by *1%

[tronical]

I think some-float * 1% should not produce an error but create a percentage value, that would be consistent with the rest of the unit handling - as opposed to having a special function just for percent.

[Murmele]

I think some-float * 1% should not produce an error but create a percentage value, that would be consistent with the rest of the unit handling - as opposed to having a special function just for percent.

Fine for me. I will check it out next days