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Re^3: Perl Cannot Be Parsed: A Formal Proof

by moritz (Cardinal)
on Jan 22, 2008 at 13:31 UTC ( #663570=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Perl Cannot Be Parsed: A Formal Proof
in thread Perl Cannot Be Parsed: A Formal Proof

Actually Perl 6 is even harder to (statically) parse than Perl 5, because it is much easier to change the syntax.

You can, for example, introduce new operators very easily:

multi sub prefix:<foo> (Str $a) { return $a eq "foo" ?? "bar" !! "foo"; }

This will define an unary prefix operator that takes one string as an argument. When somebody writes

foo "foo";

You can't know if that's a method call or a call to an operator. You'd think it makes no difference - but wait until precedence comes into the play:

BEGIN { eval " multi * prefix:<foo> (Str $a) is tighter(&infix:<~>) { return $a eq 'foo' ?? 'bar' !! 'foo'; "; } # and later in the code: foo "fo" ~ "o";

If that foo parsed as a sub call the precedence is lower than that of the concatenation ~, and the other way round if it's parsed as an operator.

This is quite a complicated example, but there are much easier ones with "real" macros. (But since no implemenation really implements macros by now, I don't really know much about them).


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