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# You may distribute under the terms of either the GNU General Public License |
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# or the Artistic License (the same terms as Perl itself) |
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# |
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# (C) Paul Evans, 2021 -- leonerd@leonerd.org.uk |
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package Feature::Compat::Try 0.04; |
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use v5.14; |
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use warnings; |
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use feature (); |
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use constant HAVE_FEATURE_TRY => defined $feature::feature{try}; |
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1809
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=head1 NAME |
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C - make C syntax available |
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=head1 SYNOPSIS |
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use Feature::Compat::Try; |
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sub foo |
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{ |
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try { |
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attempt_a_thing(); |
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return "success"; |
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} |
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catch ($e) { |
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warn "It failed - $e"; |
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return "failure"; |
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} |
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} |
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=head1 DESCRIPTION |
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This module is written in preparation for when perl will gain true native |
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syntax support for C control flow. |
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Perl added such syntax in the development version 5.33.7, which is enabled |
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by |
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use feature 'try'; |
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On that version of perl or later, this module simply enables the core feature |
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equivalent to using it directly. On such perls, this module will install with |
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no non-core dependencies, and requires no C compiler. |
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On older versions of perl before such syntax is available, it is currently |
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provided instead using the L module, imported with a |
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special set of options to configure it to recognise exactly and only the same |
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syntax as the core perl feature, thus ensuring that any code using it will |
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still continue to function on that newer perl. |
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=cut |
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=head1 KEYWORDS |
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=head2 try |
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try { |
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STATEMENTS... |
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} |
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... |
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A C statement provides the main body of code that will be invoked, and |
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must be followed by a C statement. |
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Execution of the C statement itself begins from the block given to the |
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statement and continues until either it throws an exception, or completes |
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successfully by reaching the end of the block. |
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The body of a C block may contain a C expression. If executed, |
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such an expression will cause the entire containing function to return with |
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the value provided. This is different from a plain C block, in which |
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circumstance only the C itself would return, not the entire function. |
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The body of a C block may contain loop control expressions (C, |
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C, C) which will have their usual effect on any loops that the |
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C block is contained by. |
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The parsing rules for the set of statements (the C block and its |
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associated C) are such that they are parsed as a self-contained |
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statement. Because of this, there is no need to end with a terminating |
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semicolon. |
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Even though it parses as a statement and not an expression, a C block can |
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still yield a value if it appears as the final statement in its containing |
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C or C block. For example: |
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my $result = do { |
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try { attempt_func() } |
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catch ($e) { "Fallback Value" } |
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}; |
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=head2 catch |
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... |
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catch ($var) { |
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STATEMENTS... |
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} |
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A C statement provides a block of code to the preceding C |
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statement that will be invoked in the case that the main block of code throws |
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an exception. A new lexical variable is created to store the exception in. |
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Presence of this C statement causes any exception thrown by the |
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preceding C block to be non-fatal to the surrounding code. If the |
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C block wishes to optionally handle some exceptions but not others, it |
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can re-raise it (or another exception) by calling C in the usual manner. |
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As with C, the body of a C block may also contain a C |
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expression, which as before, has its usual meaning, causing the entire |
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containing function to return with the given value. The body may also contain |
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loop control expressions (C, C or C) which also have their |
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usual effect. |
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=cut |
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sub import |
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{ |
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if( HAVE_FEATURE_TRY ) { |
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feature->import(qw( try )); |
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require warnings; |
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warnings->unimport(qw( experimental::try )); |
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} |
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else { |
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require Syntax::Keyword::Try; |
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Syntax::Keyword::Try->VERSION( '0.22' ); |
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Syntax::Keyword::Try->import(qw( try -no_finally -require_var )); |
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} |
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} |
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=head1 COMPATIBILITY NOTES |
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This module may use either L or the perl core C |
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feature to implement its syntax. While the two behave very similarly, and both |
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conform to the description given above, the following differences should be |
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noted. |
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=over 4 |
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=item * Visibility to C |
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The C module implements C blocks by using C |
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frames. As a result, they are visible to the C function and hence to |
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things like C when viewed as stack traces. |
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By comparison, core's C creates a new kind of context stack |
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entry that is ignored by C and hence these blocks do not show up in |
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stack traces. |
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This should not matter to most use-cases - e.g. even C will be |
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fine here. But if you are using C with calculated indexes to inspect |
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the state of callers to your code and there may be C frames in the way, |
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you will need to somehow account for the difference in stack height. |
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=back |
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=cut |
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=head1 AUTHOR |
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Paul Evans |
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=cut |
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0x55AA; |