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.

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(try_value) );
    
       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:

      * 

         catch ($var isa Class)

      The block is invoked only if the caught exception is a blessed
      object, and derives from the given package name.

      On Perl version 5.32 onwards, this condition test is implemented
      using the same op type that the core $var isa Class syntax is
      provided by and works in exactly the same way.

      On older perl versions it is emulated by a compatibility function.
      Currently this function does not respect a ->isa method overload on
      the exception instance. Usually this should not be a problem, as
      exception class types rarely provide such a method.

      * 

         catch ($var =~ m/regexp/)

      The block is invoked only if the caught exception is a string that
      matches the given regexp.

    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.

VALUE SEMANTICS

      Warning: the feature described in this section is experimental. This
      experiment has existed for a while, though given that since version
      0.22 the regular try syntax already behaves fine inside a do block,
      there is no longer any reason for this experimental feature to exist.
      It will start printing deprecation warnings in a later version, and
      eventually will be removed.

      Additionally, on perl versions 5.18 and later, it will produce a
      warning in the experimental category.

    The syntax provided by this module may be used as a value-yielding
    expression. Because this syntax is new, experimental, and somewhat
    surprising, it must be specifically requested by name try_value:

       use Syntax::Keyword::Try qw( try try_value );
    
       my $result = try do { ... } catch { ... };

    Also, on Perl versions 5.24 and later:

       my $result = try do { ... } finally { ... };
    
       my $result = try do { ... } catch { ... } finally { ... };

    Specifically, note that the expression must be spelled as try do { ...
    } so that the syntax is distinct from that used by control-flow
    statements. The interposed do keyword reminds the reader, and instructs
    the syntax parser, that this will be an expression, not a statement. It
    is not necessary to similarly notate the catch or finally blocks.

    In this case, the syntax behaves syntactically like an expression, and
    may appear anywhere a normal expression is allowed. It follows similar
    semantics to the purely control-flow case; if the code in the try block
    does not throw an exception, then the expression as a whole yields
    whatever value the try expression did. If it fails, then the catch
    block is executed and the expression yields its resulting value
    instead. A finally block, if present, will be evaluated for
    side-effects before the rest of the expression returns.

    Remember that, as in the control-flow case, the return keyword will
    cause the entire containing function to return, not just the try block.

OTHER MODULES

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

      * Try

      * TryCatch

      * Try::Tiny

      * Syntax::Feature::Try

    In addition, core perl itself gained a try/catch syntax based on this
    module at developemnt version 5.33.7, and should become generally
    available when 5.34 is released. It will be 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 when the
    experimental try_value feature described above has not been enabled.
    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>

