Subtracting Unit values behaves differently between 'in' and 'inch'

[christianpetty]

This could be something on my end, but I'm noticing that 'in' is not always being processed as 'inch' in arithmetic, and instead is attempting the 'to' function. Seems to only occur with negative values or subtraction.

Works:
https://math.rad.cm/?title=&precision=6&expression=-3+inch+-+4+inch

Does not work:
https://math.rad.cm/?title=&precision=6&expression=-3+in+-+4+in

[christianpetty]

More interesting cases:

Works:
https://math.rad.cm/?title=&precision=6&expression=0+mm+-+3+in+-+4+in
https://math.rad.cm/?title=&precision=6&expression=-+3+mm+-+4+in

Does not work:
https://math.rad.cm/?title=&precision=6&expression=-+3+in+-+4+in

[josdejong]

There is indeed a naming conflict between in meaning inch, or meaning to (converting a unit).

The best way to circumvent this naming conflict is to not use in, and use inch and to instead 😄

Concretely, I see one issue that may be improved upon, handling a negative value:

3in + 4in # 7 inch
-3in + 4in # error

We can do that by adding another condition in the expression parser here:

mathjs/src/expression/parse.js

Line 1062 in c35a801

(state.token === 'in' && isConstantNode(node)) ||

Making it recon with a negative values too:

(state.token === 'in' && isConstantNode(node)) || 
(state.token === 'in' && isOperatorNode(node) && node.fn === 'unaryMinus' && isConstantNode(node.args[0])) ||

I have to think though though if this really does address this concern and whether it doesn't introduce new issues. Anyone any thoughts in this regard?

[christianpetty]

Thanks! I had a feeling it might be that. No issues using inch for now. Not an option at this point for backwards compatibility, but 'to' alone could probably cover the conversion instead of 'to' and 'in'.