8.1.2 One Macro Call

Let's proceed on the interaction between active characters and macros with this small macro, which just returns its first argument:

     define([car], [$1])

The two pairs of quotes above are not part of the arguments of define; rather, they are understood by the top level when it tries to find the arguments of define. Therefore, assuming car is not already defined, it is equivalent to write:

     define(car, $1)

But, while it is acceptable for a configure.ac to avoid unnecessary quotes, it is bad practice for Autoconf macros which must both be more robust and also advocate perfect style.

At the top level, there are only two possibilities: either you quote or you don't:

     car(foo, bar, baz)
     =>foo
     [car(foo, bar, baz)]
     =>car(foo, bar, baz)

Let's pay attention to the special characters:

     car(#)
     error-->EOF in argument list

The closing parenthesis is hidden in the comment; with a hypothetical quoting, the top level understood it this way:

     car([#)]

Proper quotation, of course, fixes the problem:

     car([#])
     =>#

Here are more examples:

     car(foo, bar)
     =>foo
     car([foo, bar])
     =>foo, bar
     car((foo, bar))
     =>(foo, bar)
     car([(foo], [bar)])
     =>(foo
     define([a], [b])
     =>
     car(a)
     =>b
     car([a])
     =>b
     car([[a]])
     =>a
     car([[[a]]])
     =>[a]

With this in mind, we can explore the cases where macros invoke macros...