Hi all, I’m a relatively new and now enthusiastic Julia user. While exploring the metaprogramming capabilities I came to the question of this topic. Probably I am missing something or misinterpreting the use intention for Meta.parse and eval, but it looks like there is a big penalty for using them: …

The most basic answer is that you should almost never represent expressions as strings. Unless you are trying to write an interpreter, there is almost always a better way to solve the problem.

If you want to represent a piece of code you would be better off with Exprs. That removes the need to ensure correct syntax, and a lot of string processing (which would probably use regex, so not very performant anyway). Of course, if you are receiving string expressions from user input or files you…

Thanks for the answers! I thought that both eval and Meta.parse were pretty common in metaprogramming techniques. I understand now that Meta.parse could be expensive due to string processing (although "1+1" doesn’t seem too demanding). However, intuitively (and with no expertise whatsoever) I’d say…

To the contrary, eval and parse are some of the least-used tools in Julia’s metaprogramming arsenal. When eval is used, it’s done so at top level to —for example — define a whole slew of methods and functions over a loop of types and names. This is done up-front, where a few hundred microseconds or…

Hi again. After a break I’d like to come back to this topic with a couple more questions. If I define this simple macro: macro parse(s) Meta.parse(s) end then my naive understanding of macros would lead me to believe that running @parse "1+1" would be equivalent to eval(Meta.parse("1+1")). H…

It’s not so much about performance as about when this happens. The macro gets expanded during parsing, exactly once, and thereafter it’s as if you’d written its result yourself, which is fast: julia> @macroexpand @parse "1+1" :(1 + 1) julia> @btime macroexpand(Main, :(@parse "1+1")) 49.511 μs (1…

I see. So, do I understand correctly that I was misinterpreting the measurement of @btime @parse "1+1"? If I understand it correctly now, the actual parsing happens only once in the macro expansion, and the result of @btime is in effect like directly calling @btime 1+1.Is that correct?

Yeah. That’s why macros are the recommended way of doing metaprogramming. They take the expensive parsing stuff and move it to compile time.

When seeing benchmark timings of less than a nanosecond (like you had above), it is a sign that whatever is being benchmarked does not perform any computation at all. It usually happens because the result can be deduced right away from the expression you’re giving it. Even if you’re not familiar wi…

[image] brucala: Does this mean that metaprogramming techniques are typically avoided in performance-critical programs? Metaprogramming is something you do at parse/compile-time, not at runtime for time-critical code. If you are thinking of parse and eval as runtime techniques, you are doing…

Performance of `Meta.parse` and `eval`

General Usage Performance

Topic		Replies	Views
Prevent interpolation when Meta.parse a String to Expr New to Julia question , metaprogramming , meta	9	856	February 15, 2021
Boilerplate code generation by evaluation of parsed strings General Usage	4	723	February 26, 2018
Converts the given string into code with metaprogramming General Usage metaprogramming , strings	6	843	April 2, 2022
Macros vs. Functions with @eval(return ) General Usage macros , metaprogramming	8	3365	July 12, 2019
Evaluation of string with local variable General Usage	5	950	August 8, 2018

Performance of `Meta.parse` and `eval`

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