note Jeffrey Kegler <p>In a series of three articles in <i>The Perl Review</i>, [href://http://www.jeffreykegler.com/Home/perl-and-undecidability|now available online], I laid this proof out much more carefully, in three different versions. The most bullet-proof version derives the result directly from Rice's Theorem. <p>You are right that the proof would not go through if the only way of establishing a prototype for a Perl function was via a function definition. As I said in <i>Perl Review</i> (4.3, p. 28): <blockquote>I must show that I can use Turing-complete Perl code to determine the prototype of the <tt>dunno</tt> subroutine at compile time. A function definition won’t work.</blockquote> <p>Summarizing from that article, there are at least two ways that the prototype of a subroutine can be established using Turing-complete Perl code. One is with symbol table manipulation, for example: <blockquote><c>BEGIN { *dunno = sub () { 3 } }</c></blockquote> A second way is to put the function definition into a string which is <tt>eval</tt>'d in a BEGIN block. I believe clever monks will be able to think of others. 663393 788460