Tail recursion for preprocessor macros
9 comments · 85% upvoted · source · other discussions
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tstanisl [OP]
aalmkainzi
really cool. we will finally be able to iterate over __VA_ARGS__
flatfinger
I wonder if there would have been any difficulty simply saying that a macro invocation with N arguments is allowed to call itself recursively with any smaller number of arguments, or having a syntax to allow macro overloading based on argument count? Allowing those two things would have made many operations very simple.
Jinren
The same author is also working on a project to separate the definition of the preprocessor from the C Standard entirely, which would have several neat implications going forward.
torsten_dev
I like it.
Now how about standardizing [[clang::musttail]] so we can get some guaranteed diagnostic if a tail call does not get optimized.
Jinren
workin' on it, workin' on it (section 5)
Since it's a TS, you should get this before the next major version, too!
GCC recently added it as [[gnu::musttail]] so there's no longer any dispute among the Big Compilers that this is deliverable (apparently they just kinda forgot to do it for several years)
torsten_dev
Interesting Interesting. I do take issue with _Self. I'd prefer _Recur.
That's more in line with existing practices in functional languages and will not confuse the object oriented guys needlessly.
EDIT: author seemed receptive to changing it. Big ups.
I like the simplicity of proposed extension. When
__VA_ARGS__is non-empty, just replace__VA_TAIL__(FOO)withFOO(__VA_ARGS__)and expand it even withinFOOmacro. Infinite recursion is prevented by requiringFOOmacro to have at least one named parameter. So one parameter is consumed each time thus the recursion will end (hopefully). Moreover,__VA_ARGS__is not rescanned reducing expansion complexity from quadratic to linear.aalmkainzi 4 points · Aug 2024
does it always shift one parameter? from my understanding it shifts based on the number of named arguments, for example:
tstanisl [OP] 2 points · Aug 2024
I does not shift. However the macro that is a parameter to
__VA_TAIL__must have at least one named parameter thus...shrinks with every expansion.It should prevent infinite recursion .. at least in a typical use case.