(autoconf.info)Quotation and Nested Macros


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8.1.4 Quotation and Nested Macros
---------------------------------

The examples below use the following macros:

     define([car], [$1])
     define([active], [ACT, IVE])
     define([array], [int tab[10]])

   Each additional embedded macro call introduces other possible
interesting quotations:

     car(active)
     =>ACT
     car([active])
     =>ACT, IVE
     car([[active]])
     =>active

   In the first case, the top level looks for the arguments of `car',
and finds `active'.  Because M4 evaluates its arguments before applying
the macro, `active' is expanded, which results in:

     car(ACT, IVE)
     =>ACT

In the second case, the top level gives `active' as first and only
argument of `car', which results in:

     active
     =>ACT, IVE

i.e., the argument is evaluated _after_ the macro that invokes it.  In
the third case, `car' receives `[active]', which results in:

     [active]
     =>active

exactly as we already saw above.

   The example above, applied to a more realistic example, gives:

     car(int tab[10];)
     =>int tab10;
     car([int tab[10];])
     =>int tab10;
     car([[int tab[10];]])
     =>int tab[10];

Huh?  The first case is easily understood, but why is the second wrong,
and the third right?  To understand that, you must know that after M4
expands a macro, the resulting text is immediately subjected to macro
expansion and quote removal.  This means that the quote removal occurs
twice--first before the argument is passed to the `car' macro, and
second after the `car' macro expands to the first argument.

   As the author of the Autoconf macro `car', you then consider it to
be incorrect that your users have to double-quote the arguments of
`car', so you "fix" your macro.  Let's call it `qar' for quoted car:

     define([qar], [[$1]])

and check that `qar' is properly fixed:

     qar([int tab[10];])
     =>int tab[10];

Ahhh!  That's much better.

   But note what you've done: now that the result of `qar' is always a
literal string, the only time a user can use nested macros is if she
relies on an _unquoted_ macro call:

     qar(active)
     =>ACT
     qar([active])
     =>active

leaving no way for her to reproduce what she used to do with `car':

     car([active])
     =>ACT, IVE

Worse yet: she wants to use a macro that produces a set of `cpp' macros:

     define([my_includes], [#include <stdio.h>])
     car([my_includes])
     =>#include <stdio.h>
     qar(my_includes)
     error-->EOF in argument list

   This macro, `qar', because it double quotes its arguments, forces
its users to leave their macro calls unquoted, which is dangerous.
Commas and other active symbols are interpreted by M4 before they are
given to the macro, often not in the way the users expect.  Also,
because `qar' behaves differently from the other macros, it's an
exception that should be avoided in Autoconf.


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