Re: is lex useful?

In article 96-06-129,
>[Yup, that's what I said. Fortran needs a multi-pass lexer to correctly
>recognize that REAL*4HELLO doesn't contain the string constant 'ELLO'. -John]


A poor example. A lexer can recognize this case in a single
left-to-right scan with one character lookahead. The sequence


        letter+
        *
        digit+


at the start of a statement can be followed only by an identifier.


A better example is


            DO10I = expr1, expr2


Since the length of expr1 is bounded only by the number of characters
allowed in a statement, either a multipass lexer or practically
unbounded lookahead are needed.


Because Fortran limits the maximum size of a statement, a lexer for
Fortran can analyze any Fortran statement in constant time.


Sincerely,
Bob Corbett
[Right, thanks for the correction. In the DO10I example, note that just
looking ahead for a comma isn't sufficient. You have to look for a comma
not enclosed in parens, which lex can't do, because REs can't count. -John]


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