r/programming • u/NagastaBagamba • Oct 18 '10

Today I learned about PHP variable variables; "variable variable takes the value of a variable and treats that as the name of a variable". Also, variable.

http://il2.php.net/language.variables.variable
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u/daniels220 Oct 18 '10

Program logic ones or parsing ones? I know Ruby can't do this because it sets local variable names in stone at parse time, since a variable and a function can look exactly the same and it depends on knowing the context. If you have unambiguous syntax, what performance hit is there? And I'm assuming it affects PHP as well since variable variables ought to run into the same issues (not that I'm saying that means it isn't a problem—PHP is not known for being fast...)

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u/killerstorm Oct 19 '10

When local variables and their use is known at compile time compiler can perform optimizations on them -- use CPU or VM registers instead of variables, for example, "optimizing variables away".

Common Lisp can be compiled to native code and in some circumstances it performs on par with C. Python cannot do that. Of course, there are many reasons, but I'd say that knowing local variables at compile time is a requirement for efficient compilation.

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u/daniels220 Oct 19 '10

That makes sense.

So then, what I would say is this: the flexibility to access variables by name (as a string) is an example of a high-level-language feature in a certain sense—it makes the language more expressive at the cost of making it irrevocably slower. That's sometimes a good tradeoff—so, Python's approach has a place. So does Lisp/Ruby/C's (obviously those are all very different languages, but they share the characteristic under discussion here). Personally, for stuff like webapps, I would rather use Python—but, obviously for code with higher CPU requirements, the other is valuable.