(m4.info.gz) Input processing

 
 2.5 How `m4' copies input to output
 ===================================
 
 As `m4' reads the input token by token, it will copy each token
 directly to the output immediately.
 
    The exception is when it finds a word with a macro definition.  In
 that case `m4' will calculate the macro's expansion, possibly reading
 more input to get the arguments.  It then inserts the expansion in front
 of the remaining input.  In other words, the resulting text from a macro
 call will be read and parsed into tokens again.
 
    `m4' expands a macro as soon as possible.  If it finds a macro call
 when collecting the arguments to another, it will expand the second
 call first.  For a running example, examine how `m4' handles this input:
 
      format(`Result is %d', eval(`2**15'))
 
 First, `m4' sees that the token `format' is a macro name, so it
 collects the tokens `(', ``Result is %d'', `,', and ` ', before
 encountering another potential macro.  Sure enough, `eval' is a macro
 name, so the nested argument collection picks up `(', ``2**15'', and
 `)', invoking the eval macro with the lone argument of `2**15'.  The
 expansion of `eval(2**15)' is `32768', which is then rescanned as the
 five tokens `3', `2', `7', `6', and `8'; and combined with the next
 `)', the format macro now has all its arguments, as if the user had
 typed:
 
      format(`Result is %d', 32768)
 
 The format macro expands to `Result is 32768', and we have another
 round of scanning for the tokens `Result', ` ', `is', ` ', `3', `2',
 `7', `6', and `8'.  None of these are macros, so the final output is
 
      =>Result is 32768
 
    The order in which `m4' expands the macros can be explored using the
  Trace facilities of GNU `m4'.
 
    This process continues until there are no more macro calls to expand
 and all the input has been consumed.
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