simplify(expr, rules, {}, { exactFraction: true }) not working for (only) custom rules.

[JacobBeaudry]

Hi Jos!

Currently something like:

simplify('x * 0.25', [ '0 + n -> n' ], {}, { exactFractions: true } )

returns x * 0.25, whereas:

simplify('x * 0.25', {} , { exactFractions: true } )

or

simplify('x * 0.25', [...simplify.rules, '0 + n -> n'], {} , { exactFractions: true } )

returns x * 1 / 4.

That is, it seems as though the exactFractions option does not work if one only invokes simplify with an array of custom rules, which suggests that the conversion to exact fractions is (may be) currently implemented using the built-in simplifications rules.

Is it possible to get exact fractions when only applying custom simplification rules?

Thanks in advance! Hope all's well on your end!

Jacob

[josdejong]

The exactFractions option is an option of one of the built-in rules (a function rule simplifyConstant). If you clear all rules, the rule handling exactFractions is not executed. This rule currently not exposed and only used internally in simplify.

As a workaround, you can find the simplifyConstant from simplify.rules since it is (currently) the only function rule:

const simplifyConstant = math.simplify.rules.find(rule => typeof rule === 'function')

However this is not guaranteed to work in the future.

So I think we have to do the following

1. Improve the docs of exactFractions to explain that when you clear out all rules, that this option is not executed
2. Expose the rule somehow so you can use it on your own. Expose the factory function createSimplifyConstant, or maybe expose a new property math.simplify.simplifyConstant or so.
3. We could implement warnings or so when using exactFractions whilst simplifyConstant is missing from the rules, but that feels a bit far-fetched to me.

Yeah all going fine here, thanks for asking :)