Finding out the number of needed macro arguments

Question

How can I find out how many arguments (i.e. {} pairs) are used when 'calling' a macro? (my favorite error, that I extend a macro, i.e. it needs one more argument, then I use it to compile a text prepared for a macro having one less argument. I receive a variety of errors, depending of the environment of the macro call in the text.)

In other words: can I find out within the macro, whether it was called with the right number of {} arguments?

Answer 1

Depending on how you define "number of macro arguments" the answer may or may not be obtainable from tex and you may need to rely on the package documentation.

If you do

\newcommand\fooa[2]{abc}

\show\fooa

you see

> \fooa=\long macro:
#1#2->abc.

So you could take this #1#2-> as one definition of the command taking two {} arguments

but if you do

\newcommand\foob[2][]{abc}

\show\foob

Then you may document that as being one optional [] argument and one {} argument but \show shows the primitive definition:

> \foob=macro:
->\@protected@testopt \foob \\foob {}.

\foob technically takes no arguments at all.

Similarly after

\newcommand\fooc{\fooa}

Then \fooc operates in almost all contexts just like \fooa and acts as if it takes two arguments but

\show\fooc

shows

> \fooc=\long macro:
->\fooa .

confirming that it has no arguments at all as a primitive control sequence definition.