support comparison with zero

[aplavin]

This is convenient and unambiguous when constrained for scalar units

[codecov-commenter]

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@@            Coverage Diff             @@
##           master     #640      +/-   ##
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- Coverage   89.06%   89.02%   -0.05%     
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+ Hits         1328     1330       +2     
- Misses        163      164       +1     

Impacted Files	Coverage Δ
src/quantities.jl	93.97% <66.66%> (-0.34%)	⬇️

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[cstjean]

By the same token, you could support addition by 0 and equality with 0. But unfortunately,

julia> 0u"m" == 0
false

so that'd be a breaking change.

[aplavin]

Equality with zero is already convenient and perfectly general in Julia: iszero(some_unitful_function()). It doesn't require any further operators, especially breaking.

As isless is bound with isequal according to their docstring, I agree that isless shouldn't be defined here. Can remove it, thanks for pointing out!
But there's no restriction on <, and being able to do some_unitful_function() > 0 would be an improvement.

[cstjean]

But there's no restriction on <, and being able to do some_unitful_function() > 0 would be an improvement.

I agree that it's convenient. I just worry about general consistency. You'd have that 0m < 0, 0m > 0, 0m == 0 all false. 0 <= 0m would also be false, which is particularly uncomfortable.

[aplavin]

0 <= 0m would also be false

Sorry, that's just an oversight! Can easily add <= as well. It's also non-breaking, same as <.

[cstjean]

Yeah, but then you'd have that x less-than-or-equal y is not the same as x<y || x==y

[sostock]

Some previous discussion: #455

Another point to consider: it feels inconsistent to me that 0 < 1m holds, but 0s < 1m doesn’t.

But it would be nice to have a more convenient method of comparing with zero.