4.5 Renaming macros
It is possible to rename an already defined macro. To do this, you need
the builtin `defn':
-- Builtin: defn (NAME)
Expands to the _quoted definition_ of NAME. If the argument is
not a defined macro, the expansion is void.
If NAME is a user-defined macro, the quoted definition is simply
the quoted expansion text. If, instead, NAME is a builtin, the
expansion is a special token, which points to the builtin's
internal definition. This token is only meaningful as the second
argument to `define' (and `pushdef'), and is silently converted to
an empty string in most other contexts.
The macro `defn' is recognized only with parameters.
Its normal use is best understood through an example, which shows
how to rename `undefine' to `zap':
In this way, `defn' can be used to copy macro definitions, and also
definitions of builtin macros. Even if the original macro is removed,
the other name can still be used to access the definition.
The fact that macro definitions can be transferred also explains why
you should use `$0', rather than retyping a macro's name in its
define(`foo', `This is `$0'')
=>This is bar
Macros used as string variables should be referred through `defn',
to avoid unwanted expansion of the text:
define(`string', `The macro dnl is very useful
=>The macro defn(`string')
=>The macro dnl is very useful
However, it is important to remember that `m4' rescanning is purely
textual. If an unbalanced end-quote string occurs in a macro
definition, the rescan will see that embedded quote as the termination
of the quoted string, and the remainder of the macro's definition will
be rescanned unquoted. Thus it is a good idea to avoid unbalanced
end-quotes in macro definitions or arguments to macros.
Using `defn' to generate special tokens for builtin macros outside
of expected contexts can sometimes trigger warnings. But most of the
time, such tokens are silently converted to the empty string.
define(defn(`divnum'), `cannot redefine a builtin token')
error-->m4:stdin:2: Warning: define: invalid macro name ignored
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