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6.2 If-else construct, or multibranch

The other conditional, `ifelse', is much more powerful.  It can be used
as a way to introduce a long comment, as an if-else construct, or as a
multibranch, depending on the number of arguments supplied:

 -- Builtin: ifelse (COMMENT)
 -- Builtin: ifelse (STRING-1, STRING-2, EQUAL, [NOT-EQUAL])
 -- Builtin: ifelse (STRING-1, STRING-2, EQUAL-1, STRING-3, STRING-4,
          EQUAL-2, ..., [NOT-EQUAL])
     Used with only one argument, the `ifelse' simply discards it and
     produces no output.

     If called with three or four arguments, `ifelse' expands into
     EQUAL, if STRING-1 and STRING-2 are equal (character for
     character), otherwise it expands to NOT-EQUAL.  A final fifth
     argument is ignored, after triggering a warning.

     If called with six or more arguments, and STRING-1 and STRING-2
     are equal, `ifelse' expands into EQUAL-1, otherwise the first
     three arguments are discarded and the processing starts again.

     The macro `ifelse' is recognized only with parameters.

   Using only one argument is a common `m4' idiom for introducing a
block comment, as an alternative to repeatedly using `dnl'.  This
special usage is recognized by GNU `m4', so that in this case, the
warning about missing arguments is never triggered.

     ifelse(`some comments')
     ifelse(`foo', `bar')
     error-->m4:stdin:2: Warning: too few arguments to builtin `ifelse'

   Using three or four arguments provides decision points.

     ifelse(`foo', `bar', `true')
     ifelse(`foo', `foo', `true')
     define(`foo', `bar')
     ifelse(foo, `bar', `true', `false')
     ifelse(foo, `foo', `true', `false')

   Notice how the first argument was used unquoted; it is common to
compare the expansion of a macro with a string.  With this macro, you
can now reproduce the behavior of blind builtins, where the macro is
recognized only with arguments.

     define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')')
     foo(`a', `b', `c')

   For an example of a way to make defining blind macros easier, see
Note: Composition.

   The macro `ifelse' can take more than four arguments.  If given more
than four arguments, `ifelse' works like a `case' or `switch' statement
in traditional programming languages.  If STRING-1 and STRING-2 are
equal, `ifelse' expands into EQUAL-1, otherwise the procedure is
repeated with the first three arguments discarded.  This calls for an

     ifelse(`foo', `bar', `third', `gnu', `gnats')
     error-->m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored
     ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth')
     ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh')
     ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8')
     error-->m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored

   Naturally, the normal case will be slightly more advanced than these
examples.  A common use of `ifelse' is in macros implementing loops of
various kinds.

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