(m4.info.gz) Defn

Info Catalog

(m4.info.gz) Undefine

(m4.info.gz) Definitions

(m4.info.gz) Pushdef

 
 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':
 
      define(`zap', defn(`undefine'))
      =>
      zap(`undefine')
      =>
      undefine(`zap')
      =>undefine(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
 definition:
 
      define(`foo', `This is `$0'')
      =>
      define(`bar', defn(`foo'))
      =>
      bar
      =>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
      ')
      =>
      string
      =>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.
 
      define(`foo', a'a)
      =>
      define(`a', `A')
      =>
      define(`echo', `$@')
      =>
      foo
      =>A'A
      defn(`foo')
      =>aA'
      echo(foo)
      =>AA'
 
    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.
 
      defn(`defn')
      =>
      define(defn(`divnum'), `cannot redefine a builtin token')
      error-->m4:stdin:2: Warning: define: invalid macro name ignored
      =>
      divnum
      =>0
 

Info Catalog

(m4.info.gz) Undefine

(m4.info.gz) Definitions

(m4.info.gz) Pushdef