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Re^2: Perl Cannot Be Parsed: A Formal Proof

by Anonymous Monk
on Jan 28, 2008 at 01:48 UTC ( #664604=note: print w/ replies, xml ) Need Help??


in reply to Re: Perl Cannot Be Parsed: A Formal Proof
in thread Perl Cannot Be Parsed: A Formal Proof

This is nonsense. Eval does not preclude parsing, unless results of evaluating something can alter the parse of another part.


Comment on Re^2: Perl Cannot Be Parsed: A Formal Proof
Re^3: Perl Cannot Be Parsed: A Formal Proof
by Anonymous Monk on Jan 28, 2008 at 03:08 UTC
    There are many languages that have "eval" and yet have unambiguous parse trees for the actual code of the program (as opposed to the code evaluated at runtime).

    If you run this Python program you'll see it prints out a complete parse tree of the program BEFORE it runs eval. Before you jump down my throat, I'm not saying that this makes Python a better language than Perl. It's just a fact that a language can be constructed so that "eval" is totally handled at runtime and does not affect the parsability of the program.

    import sys import parser print parser.suite(open(__file__).read()).totuple() x = eval(sys.stdin.readline()) if x: print "yes", x else: print "no", x
    To bring this back to the original post, the "parser" module is the Python equivalent of what Parse::Perl would be if Perl parsing were possible.
Re^3: Perl Cannot Be Parsed: A Formal Proof
by perlfan (Curate) on Jan 28, 2008 at 03:47 UTC
    He is saying that in order to statically parse an eval block, you must know when_it_halts - or in otherwords - what it does; this is impossible with out actually running the eval block, and is clearly related to the halting problem.

      If he's saying that, then he's wrong. The problem is not the existence of eval; the problem in Perl 5 is barewords and prototypes.

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