Next: Quoting and Parameters, Previous: Active Characters, Up: M4 Quotation
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]