Disable overriding existing units, functions, constants when assigning a symbol

[arve0]

Hi!
I expected that the unit m for meter not to be overridden by scope when using math.eval.

Example in REPL:

$ node bin/repl.js
> scope = {}
{}
> math.eval("m = 10kg", scope).toString()
'10 kg'
> math.eval("v = 10m/s", scope).toString()
'100 kg / s' // <-- expected this to be 10 m/s

I guess this a weakness with eval because:

1. I can do math.unit(10, 'm/s').
2. It is hard to distinguish a valid multiplication vs number + unit.

To give it some context, I wanted to do some physics calculations in a web REPL I made:

Of course I could call the mass mass instead of m, but as this is quite a common way to name symbols in physics I guess others might fall in the same trap.

Any ideas? Could the "unit-syntax" be in some other way? For example:

1m/s  // no space -> prefer m/s as unit, do not multiply/divide if m/s in scope
1 m/s  // space -> prefer multiplication if m/s are in scope

[josdejong]

Thanks for bringing this up. Right now the expression parser doesn't know about units, for the parser everything is just a symbol that will be resolved at evaluation time by first looking in the scope, then in math, then whether it's a unit.

Your suggestion of distinguishing between variables and units isn't something obvious to implement. I don't think it's possible to sometimes interpret a symbol x in 1.23 x as a unit and other times as a variable. Distinguishing with or without space would be very confusing I'm afraid.

One option would be to give an error or when overriding a unit or built-in constant or function (like m, pi, or sqrt), and implement some way to override this restriction.

Related: #792

[arve0]

Distinguishing with or without space would be very confusing I'm afraid.

Yes, it would. What about some other syntax to force evaluation as unit first? A key word? Wrapped in parentheses?

One option would be to give an error or when overriding a unit or built-in constant or function (like m, pi, or sqrt), and implement some way to override this restriction.

Not sure how I feel loosing some of the conventional symbols of physics. In one way, F = m g is short and concise. But on the other side, I've seen students mangle up the symbols with units (and I even do it my self, time to time).

Just struck me that (after writing the above), I could force units by using it's full name:

$ node bin/repl.js
> scope = {}
{}
> math.eval("m = 10 kg", scope).toString()
'10 kg'
> math.eval("v = 10 meter / s", scope).toString()
'10 meter / s' // <-- as expected

In light of the last example, a warning would probably suffice. Is this something you would want implemented? throw if symbol is already defined as a unit?

[josdejong]

Is this something you would want implemented? throw if symbol is already defined as a unit?

Yes, having an option to disable overriding existing symbols/functions and throw an error if you try makes sense. It has bitten me too a couple of times :)