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Re^4: Perl Cannot Be Parsed: A Formal Proof

by Anonymous Monk
on Aug 13, 2009 at 20:25 UTC ( #788435=note: print w/replies, xml ) Need Help??

in reply to Re^3: Perl Cannot Be Parsed: A Formal Proof
in thread Perl Cannot Be Parsed: A Formal Proof

I agree, completely. However, the question of whether one can do what you claim remains open. Put another way, if you have an ambiguous program e_0, can you transform it into the equivalent program P

P = " if (C_0) { e_0 } elsif (C_1) { e_1 } elsif (C_2) { e_2 } ... "

where C_i are conditionals determining which way to parse e_0, and e_i are syntactic permutations of e_0 that force Perl to parse in the particular way dictated by C_i?

The broader conjecture is: Given an arbitrary e_0, there exists program transformation F(e_0) = P such that P is (a) finite, (b) parsed exactly one way by Perl, and (c) operationally-equivalent to e_0 -- e_0 and P produce equal results for equal inputs in all contexts.

This would be a useful result for compiler hackers. It says that you can take a Perl program and statically transform it into a syntax that is guaranteed to remain unchanged during execution. This property allows you to design more aggressive program transformations and optimizations, unencumbered by the confusion of "what might the interpreter do to the program structure?"

Perl is cool and all, but the fact that it isn't supported by a uniform conceptual model (small kernel of concepts from which many broader concepts are composed) will keep preventing fast compilers/interpreters and good tool support, in general.

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