(bison.info) Rpcalc Input

 
 Explanation of `input'
 ......................
 
    Consider the definition of `input':
 
      input:    /* empty */
              | input line
      ;
 
    This definition reads as follows: "A complete input is either an
 empty string, or a complete input followed by an input line".  Notice
 that "complete input" is defined in terms of itself.  This definition
 is said to be "left recursive" since `input' appears always as the
 leftmost symbol in the sequence.   Recursive Rules Recursion.
 
    The first alternative is empty because there are no symbols between
 the colon and the first `|'; this means that `input' can match an empty
 string of input (no tokens).  We write the rules this way because it is
 legitimate to type `Ctrl-d' right after you start the calculator.  It's
 conventional to put an empty alternative first and write the comment
 `/* empty */' in it.
 
    The second alternate rule (`input line') handles all nontrivial
 input.  It means, "After reading any number of lines, read one more
 line if possible."  The left recursion makes this rule into a loop.
 Since the first alternative matches empty input, the loop can be
 executed zero or more times.
 
    The parser function `yyparse' continues to process input until a
 grammatical error is seen or the lexical analyzer says there are no more
 input tokens; we will arrange for the latter to happen at end of file.
Info Catalog
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