Evaluate with strings and numbers

[thadev]

Hi, thanks for the fast eval crate. is there a way to use numbers and strings as variables? Something like this: Evaluate: x > 1 && y == 'Y' ? where x will be a number and y a string.

[likebike]

Hello thadev,

Right now, the only type of value that can exist is f64.

The 'print' statement has special parsing logic that can also handle string values, but these strings can't exist outside of a 'print' call, and they can't participate in expressions (like comparisons or addition).

It is theoretically possible to add string values, but we would need to think about the following issues:

• Right now, evaluation can always assume that everything is the same type, so it doesn't need to check for data types before performing operations. Because of this (along with many other optimizations) the library is quite fast -- compiled expressions often run at half the speed of compiled, optimized Rust which is amazing. In fact, the entire evaluation has only one 'if' statement in the critical path; even by adding a single 'if' statement into the evaluation logic, it would significantly reduce performance.
• String addition is not commutative: A+B==B+A is true for numbers but not strings. This reduces our ability to optimize expressions ahead-of-time, and we would need to add another "optimizer" that understands these string rules.
• Many arithmetic operations don't make sense for strings: subtraction, division, exponentiation, logarithm, round, etc.

It is possible to avoid the above performance problems by having two different parse results: one "pure_f64" expression type that consists only of f64, and one "mixed" expression type that has strings or other non-f64 data types. The "pure" type would maintain all of the current high performance. The "mixed" would sacrifice performance to enable string values.

In the long run, I do want to support more data types (like integer, rational, complex, etc) and it makes sense to include strings in this list. The above list of "things to think about" will need to be done for each different type that is added -- strings are no worse than any other type.

[thadev]

Hello likebike,

The performance is impressive. I would go for the solution with the two parsers - one with f64 only and one with mixed types OR at least one parser for each type.

Personally, I would use the mixed types parser for simple conditions and the f64 parser for calculation expressions. f64 is pretty versatile when it comes to calculations, so I don't miss the other types at all (integers etc.), but I definitely miss string for conditions that other crates have. So I have to stick with one of the others for the time being (at least for conditions, and I hope you will add string soon.

Thanks

[likebike]

Thanks for the feedback.