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9.2 Undiverting output

Diverted text can be undiverted explicitly using the builtin undivert:

— Builtin: undivert ([number]...)

Undiverts the diversions given by the arguments, in the order given. If no arguments are supplied, all diversions are undiverted, in numerical order. As a GNU extension, if number is not numeric, treat it as a file name instead.

The expansion of undivert is void. 

     divert(`1')
     This text is diverted.
     divert
     =>
     This text is not diverted.
     =>This text is not diverted.
     undivert(`1')
     =>
     =>This text is diverted.
     =>

Notice the last two blank lines. One of them comes from the newline following undivert, the other from the newline that followed the divert! A diversion often starts with a blank line like this.

When diverted text is undiverted, it is not reread by m4, but rather copied directly to the current output, and it is therefore not an error to undivert into a diversion. Undiverting the empty string is the same as specifying diversion 0; in either case nothing happens since the output has already been flushed. 

     divert(`1')diverted text
     divert
     =>
     undivert()
     =>
     undivert(`0')
     =>
     undivert
     =>diverted text
     =>

When a diversion has been undiverted, the diverted text is discarded, and it is not possible to bring back diverted text more than once. 

     divert(`1')
     This text is diverted first.
     divert(`0')undivert(`1')dnl
     =>
     =>This text is diverted first.
     undivert(`1')
     =>
     divert(`1')
     This text is also diverted but not appended.
     divert(`0')undivert(`1')dnl
     =>
     =>This text is also diverted but not appended.

Attempts to undivert the current diversion are silently ignored. Thus, when the current diversion is not 0, the current diversion does not get rearranged among the other diversions. 

     divert(`1')one
     divert(`2')two
     divert(`3')three
     divert(`2')undivert`'dnl
     divert`'undivert`'dnl
     =>two
     =>one
     =>three

GNU m4 allows named files to be undiverted. Given a non-numeric argument, the contents of the file named will be copied, uninterpreted, to the current output. This complements the builtin include (see Include). To illustrate the difference, the file examples/foo contains the word `bar': 

     define(`bar', `BAR')
     =>
     undivert(`foo')
     =>bar
     =>
     include(`foo')
     =>BAR
     =>

If the file is not found (or cannot be read), an error message is issued, and the expansion is void.