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Postings from June 2006
Author: larry Date: Thu Jun 15 16:05:17 2006 New Revision: 9660 Modified: doc/trunk/design/syn/S04.pod Log: Definition of warning control exceptions. Modified: doc/trunk/design/syn/S04.pod ============================================================================== --- doc/trunk/design/syn/S04.pod (original) +++ doc/trunk/design/syn/S04.pod Thu Jun 15 16:05:17 2006 @@ -12,9 +12,9 @@ Maintainer: Larry Wall <larry@wall.org> Date: 19 Aug 2004 - Last Modified: 7 June 2006 + Last Modified: 15 June 2006 Number: 4 - Version: 22 + Version: 23 This document summarizes Apocalypse 4, which covers the block and statement syntax of Perl. @@ -473,6 +473,30 @@ in the current sub was implicit. For Perl 6 we have to make this preference explicit.) +Warnings are produced in Perl 6 by throwing a resumable control +exception to the outermost scope, which by default prints the +warning and resumes the exception by extracting a resume continuation +from the exception, which must be supplied by the warn() function +(or equivalent). Exceptions are not resumable in Perl 6 unless +the exception object does the C<Resumable> role. (Note that fatal +exception types can do the C<Resumable> role even if thrown via +C<fail()>--when uncaught they just hit the outermost fatal handler +instead of the outermost warning handler, so some inner scope has to +explicitly treat them as warnings and resume them.) + +Since warnings are processed using the standard control exception +mechanism, they may be intercepted and either suppressed or fatalized +anywhere within the dynamic scope by supplying a suitable C<CONTROL> +block. This dynamic control is orthogonal to any lexically scoped +warning controls, which merely decide whether to call C<warn()> +in the first place. + +As with calls to C<return>, the warning control exception is an +abstraction that the compiler is free to optimize away (along with the +associated continuation) when the compiler or runtime can determine +that the semantics would be preserved by merely printing out the +error and going on. + =head1 The goto statement In addition to C<next>, C<last>, and C<redo>, Perl 6 also supports