CONTENTS

NAME

Syntax::Keyword::Try - a try/catch/finally syntax for perl

SYNOPSIS

use Syntax::Keyword::Try;

sub foo {
   try {
      attempt_a_thing();
      return "success";
   }
   catch ($e) {
      warn "It failed - $e";
      return "failure";
   }
}

DESCRIPTION

This module provides a syntax plugin that implements exception-handling semantics in a form familiar to users of other languages, being built on a block labeled with the try keyword, followed by at least one of a catch or finally block.

As well as providing a handy syntax for this useful behaviour, this module also serves to contain a number of code examples for how to implement parser plugins and manipulate optrees to provide new syntax and behaviours for perl code.

Syntax similar to this module has now been added to core perl, starting at version 5.34.0. If you are writing new code, it is suggested that you instead use the Feature::Compat::Try module instead, as that will enable the core feature on those supported perl versions, falling back to Syntax::Keyword::Try on older perls.

Experimental Features

Some of the features of this module are currently marked as experimental. They will provoke warnings in the experimental category, unless silenced.

You can silence this with no warnings 'experimental' but then that will silence every experimental warning, which may hide others unintentionally. For a more fine-grained approach you can instead use the import line for this module to only silence this module's warnings selectively:

use Syntax::Keyword::Try qw( try :experimental(typed) );

use Syntax::Keyword::Try qw( try :experimental );  # all of the above

Don't forget to import the main try symbol itself, to activate the syntax.

KEYWORDS

try

try {
   STATEMENTS...
}
...

A try statement provides the main body of code that will be invoked, and must be followed by either a catch statement, a finally statement, or both.

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. What will happen next depends on the presence of a catch or finally statement immediately following it.

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 and finally) 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" }
};

Note (especially to users of Try::Tiny and similar) that the try {} block itself does not necessarily stop exceptions thrown inside it from propagating outside. It is the presence of a later catch {} block which causes this to happen. A try with only a finally and no catch will still propagate exceptions up to callers as normal.

catch

...
catch ($var) {
   STATEMENTS...
}

or

...
catch {
   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. Optionally a new lexical variable can be provided to store the exception in. If not provided, the catch block can inspect the raised exception by looking in $@ instead.

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.

If a catch statement is not given, then any exceptions raised by the try block are raised to the caller in the usual way.

catch (Typed)

...
catch ($var isa Class) { ... }

...
catch ($var =~ m/^Regexp match/) { ... }

Experimental; since version 0.15.

Optionally, multiple catch statements can be provided, where each block is given a guarding condition, to control whether or not it will catch particular exception values. Use of this syntax will provoke an experimental category warning on supporting perl versions, unless silenced by importing the :experimental(typed) tag (see above).

Two kinds of condition are supported:

When an exception is caught, each condition is tested in the order they are written in, until a matching case is found. If such a case is found the corresponding block is invoked, and no further condition is tested. If no contional block matched and there is a default (unconditional) block at the end then that is invoked instead. If no such block exists, then the exception is propagated up to the calling scope.

finally

...
finally {
   STATEMENTS...
}

A finally statement provides a block of code to the preceding try statement (or 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.

Note that the finally syntax is not available when using this module via Feature::Compat::Try, as it is not expected that syntax will be added to the core perl 'try' feature. This is because a more general-purpose ability may be added instead, under the name 'defer'. If you wish to write code that may more easily be forward-compatible with that feature instead, you should consider using Syntax::Keyword::Defer rather than using finally statements.

OTHER MODULES

There are already quite a number of modules on CPAN that provide a try/catch-like syntax for Perl.

In addition, core perl itself gained a try/catch syntax based on this module at version 5.34.0. It is available as use feature 'try'.

They are compared here, by feature:

True syntax plugin

Like Try and Syntax::Feature::Try, this module is implemented as a true syntax plugin, allowing it to provide new parsing rules not available to simple functions. Most notably here it means that the resulting combination does not need to end in a semicolon.

The core feature 'try' is also implemented as true native syntax in the perl parser.

In comparison, Try::Tiny is plain perl and provides its functionality using regular perl functions; as such its syntax requires the trailing semicolon.

TryCatch is a hybrid that uses Devel::Declare to parse the syntax tree.

@_ in a try or catch block

Because the try and catch block code is contained in a true block rather than an entire anonymous subroutine, invoking it does not interfere with the @_ arguments array. Code inside these blocks can interact with the containing function's array as before.

This feature is unique among these modules; none of the others listed have this ability.

The core feature 'try' also behaves in this manner.

return in a try or catch block

Like TryCatch and Syntax::Feature::Try, the return statement has its usual effect within a subroutine containing syntax provided by this module. Namely, it causes the containing sub itself to return.

It also behaves this way using the core feature 'try'.

In comparison, using Try or Try::Tiny mean that a return statement will only exit from the try block.

next/last/redo in a try or catch block

The loop control keywords of next, last and redo have their usual effect on dynamically contained loops.

These also work fine when using the core feature 'try'.

Syntax::Feature::Try documents that these do not work there. The other modules make no statement either way.

Value Semantics

Like Try and Syntax::Feature::Try, the syntax provided by this module only works as a syntax-level statement and not an expression. You cannot assign from the result of a try block. A common workaround is to wrap the try/catch statement inside a do block, where its final expression can be captured and used as a value.

The same do block wrapping also works for the core feature 'try'.

In comparison, the behaviour implemented by Try::Tiny can be used as a valued expression, such as assigned to a variable or returned to the caller of its containing function.

try without catch

Like Syntax::Feature::Try, the syntax provided by this module allows a try block to be followed by only a finally block, with no catch. In this case, exceptions thrown by code contained by the try are not suppressed, instead they propagate as normal to callers. This matches the behaviour familiar to Java or C++ programmers.

In comparison, the code provided by Try and Try::Tiny always suppress exception propagation even without an actual catch block.

The TryCatch module does not allow a try block not followed by catch.

The core feature 'try' does not implement finally at all, and also requires that every try block be followed by a catch.

Typed catch

Try and Try::Tiny make no attempt to perform any kind of typed dispatch to distinguish kinds of exception caught by catch blocks.

Likewise the core feature 'try' currently does not provide this ability, though it remains an area of ongoing design work.

TryCatch and Syntax::Feature::Try both attempt to provide a kind of typed dispatch where different classes of exception are caught by different blocks of code, or propagated up entirely to callers.

This module provides such an ability, via the currently-experimental catch (VAR cond...) syntax.

The design thoughts continue on the RT ticket https://rt.cpan.org/Ticket/Display.html?id=123918.

WITH OTHER MODULES

Future::AsyncAwait

As of Future::AsyncAwait version 0.10 and Syntax::Keyword::Try version 0.07, cross-module integration tests assert that basic try/catch blocks inside an async sub work correctly, including those that attempt to return from inside try.

use Future::AsyncAwait;
use Syntax::Keyword::Try;

async sub attempt
{
   try {
      await func();
      return "success";
   }
   catch {
      return "failed";
   }
}

ISSUES

Thread-safety at load time cannot be assured before perl 5.16

On perl versions 5.16 and above this module is thread-safe.

On perl version 5.14 this module is thread-safe provided that it is used before any additional threads are created.

However, when using 5.14 there is a race condition if this module is loaded late in the program startup, after additional threads have been created. This leads to the potential for it to be started up multiple times concurrently, which creates data races when modifying internal structures and likely leads to a segmentation fault, either during load or soon after when more code is compiled.

As a workaround, for any such program that creates multiple threads, loads additional code (such as dynamically-discovered plugins), and has to run on 5.14, it should make sure to

use Syntax::Keyword::Try;

early on in startup, before it spins out any additional threads.

(See also https://rt.cpan.org/Public/Bug/Display.html?id=123547)

$@ is not local'ised by try do before perl 5.24

On perl versions 5.24 and above, or when using only control-flow statement syntax, $@ is always correctly localised.

However, when using the experimental value-yielding expression version try do {...} on perl versions 5.22 or older, the localisation of $@ does not correctly apply around the expression. After such an expression, the value of $@ will leak out if a failure happened and the catch block was invoked, overwriting any previous value that was visible there.

(See also https://rt.cpan.org/Public/Bug/Display.html?id=124366)

ACKNOWLEDGEMENTS

With thanks to Zefram, ilmari and others from irc.perl.org/#p5p for assisting with trickier bits of XS logic.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>