Feature::Compat::Try(3User Contributed Perl DocumentaFeature::Compat::Try(3pm)
NAME
"Feature::Compat::Try" - make "try/catch" syntax available
SYNOPSIS
use Feature::Compat::Try;
sub foo
{
try {
attempt_a_thing();
return "success";
}
catch ($e) {
warn "It failed - $e";
return "failure";
}
}
DESCRIPTION
This module makes syntax support for "try/catch" control flow easily
available.
Perl added such syntax at version 5.34.0, and extended it to support
optional "finally" blocks at 5.35.9, which is enabled by
use feature 'try';
On that version of perl or later, this module simply enables the core
feature equivalent to using it directly. On such perls, this module
will install with no non-core dependencies, and requires no C compiler.
On older versions of perl before such syntax is available, it is
currently provided instead using the Syntax::Keyword::Try module,
imported with a special set of options to configure it to recognise
exactly and only the same syntax as the core perl feature, thus
ensuring that any code using it will still continue to function on that
newer perl.
KEYWORDS
try
try {
STATEMENTS...
}
...
A "try" statement provides the main body of code that will be invoked,
and must be followed by a "catch" statement. It may optionally be
followed by a "finally" statement.
Execution of the "try" statement itself begins from the block given to
the statement and continues until either it throws an exception, or
completes successfully by reaching the end of the block.
The body of a "try {}" block may contain a "return" expression. If
executed, such an expression will cause the entire containing function
to return with the value provided. This is different from a plain "eval
{}" block, in which circumstance only the "eval" itself would return,
not the entire function.
The body of a "try {}" block may contain loop control expressions
("redo", "next", "last") which will have their usual effect on any
loops that the "try {}" block is contained by.
The parsing rules for the set of statements (the "try" block and its
associated "catch") are such that they are parsed as a self-contained
statement. Because of this, there is no need to end with a terminating
semicolon.
Even though it parses as a statement and not an expression, a "try"
block can still yield a value if it appears as the final statement in
its containing "sub" or "do" block. For example:
my $result = do {
try { attempt_func() }
catch ($e) { "Fallback Value" }
};
catch
...
catch ($var) {
STATEMENTS...
}
A "catch" statement provides a block of code to the preceding "try"
statement that will be invoked in the case that the main block of code
throws an exception. A new lexical variable is created to store the
exception in.
Presence of this "catch" statement causes any exception thrown by the
preceding "try" block to be non-fatal to the surrounding code. If the
"catch" block wishes to optionally handle some exceptions but not
others, it can re-raise it (or another exception) by calling "die" in
the usual manner.
As with "try", the body of a "catch {}" block may also contain a
"return" expression, which as before, has its usual meaning, causing
the entire containing function to return with the given value. The body
may also contain loop control expressions ("redo", "next" or "last")
which also have their usual effect.
finally
...
finally {
STATEMENTS...
}
A "finally" statement provides an optional block of code to the
preceding "try"/"catch" pair which is executed afterwards, both in the
case of a normal execution or a thrown exception. This code block may
be used to provide whatever clean-up operations might be required by
preceding code.
Because it is executed during a stack cleanup operation, a "finally {}"
block may not cause the containing function to return, or to alter the
return value of it. It also cannot see the containing function's @_
arguments array (though as it is block scoped within the function, it
will continue to share any normal lexical variables declared up until
that point). It is protected from disturbing the value of $@. If the
"finally {}" block code throws an exception, this will be printed as a
warning and discarded, leaving $@ containing the original exception, if
one existed.
COMPATIBILITY NOTES
This module may use either Syntax::Keyword::Try or the perl core "try"
feature to implement its syntax. While the two behave very similarly,
and both conform to the description given above, the following
differences should be noted.
• Visibility to "caller()"
The "Syntax::Keyword::Try" module implements "try" blocks by using
"eval" frames. As a result, they are visible to the "caller()"
function and hence to things like "Carp::longmess" when viewed as
stack traces.
By comparison, core's "feature 'try'" creates a new kind of context
stack entry that is ignored by "caller()" and hence these blocks do
not show up in stack traces.
This should not matter to most use-cases - e.g. even "Carp::croak"
will be fine here. But if you are using "caller()" with calculated
indexes to inspect the state of callers to your code and there may
be "try" frames in the way, you will need to somehow account for
the difference in stack height.
• "B::Deparse"
The core "feature 'try'" is implemented by emitting real opcodes
that represent its behaviour, which is recognised by the version of
B::Deparse that ships with core perl. As a result, any code using
this implementation will deparse currently with tools like "perl
-MO=Deparse ...", or others related to it such as coverage
checkers.
By comparison, since "Syntax::Keyword::Try" uses "OP_CUSTOM" it is
not recognised by "B::Deparse" and so attempts to deparse this will
result in error messages like
unexpected OP_CUSTOM (catch) at ...
This is rather unavoidable due to the way that "B::Deparse" is
implemented and does not easily support custom operators.
See also <https://rt.cpan.org/Ticket/Display.html?id=134812>.
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.34.0 2022-02-25 Feature::Compat::Try(3pm)
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