### 6.1 Testing if a macro is defined ¶

There are two different builtin conditionals in `m4`. The first is `ifdef`:

Builtin: ifdef(name, string-1, [string-2]) ¶

  If name is defined as a macro, `ifdef` expands to string-1, otherwise to string-2. If string-2 is omitted, it is taken to be the empty string (according to the normal rules).

  The macro `ifdef` is recognized only with parameters.

    ifdef(`foo', ``foo' is defined', ``foo' is not defined')
    ⇒foo is not defined
    define(`foo', `')
    ⇒
    ifdef(`foo', ``foo' is defined', ``foo' is not defined')
    ⇒foo is defined
    ifdef(`no_such_macro', `yes', `no', `extra argument')
    error→m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored
    ⇒no