- CONTINUATION TARGETS
- INACCESSIBLE STACK FRAMES
- CONTINUATION VALIDITY
- SEE ALSO
Scope::Escape - reified escape continuations
use Scope::Escape qw(current_escape_function); $escape = current_escape_function; ... $escape->($result); use Scope::Escape::Continuation qw(current_escape_continuation); $escape = current_escape_continuation; ... $escape->go($result);
This module provides a generalised facility for non-local control transfer (jumping between stack frames), based on the well-thought-out semantics of Common Lisp. It provides operators that will capture and reify the escape (return) continuation of the current stack frame. The stack frame can then be returned from, at (nearly) any time while it still exists, via the reified continuation. This applies not only to subroutine stack frames, but also to intermediate frames for code blocks, and other kinds of stack frame. This facility can be used directly, or wrapped up to build a more structured facility, as is done by Scope::Escape::Sugar.
The system of reified escape continuations is fundamentally different from Perl's native
die exception facility. With
die, the code initiating the non-local transfer has no control over where it will go to. Each
eval frame gets to decide whether it wants to act as the target of the thrown exception, but it must make this decision based almost entirely on what was recorded in the exception object, because the stack frames between the
die and the
eval have already been unwound by that time. With reified continuations, however, the code initiating the transfer determines where it will go to (by choosing which continuation to use), and that decision can be made with all information about the circumstances still available.
A reified escape continuation appears in Perl as a function object. Calling the function results in returning from the stack frame that is the target of the continuation. Values passed to the function are returned from the target stack frame. Optionally, the continuation may be blessed into the Scope::Escape::Continuation class. This class provides a method-based interface to the continuation: transferring through the continuation, and querying its state, can be performed by method calls on the continuation object. The methods can also be called directly, as functions, on unblessed continuation functions.
The operators supplied by this module generate continuations targeting the "current scope". It is not always obvious what that is. Here are the types of scope that occur in Perl and which can be escaped from by means of the reified continuations supplied by this module:
Any braced block of code is a scope. Escaping from it jumps to the end of the block. If the block is in a context where it supplies a value, then using the escape continuation supplies that value, as if it had been the value of the last statement executed in the block.
In the case of the
doblock syntax, the value returned from the block is used directly in the surrounding expression. Blocks in
grepalso supply a value. Some other kinds of block are mentioned specially below. In most other cases a block is in void context.
- loop statement
In a loop statement, the loop body is a block, with its own scope. The
continueblock, if any, is likewise a separate block scope. A loop iteration is also a scope, and the test expression is evaluated within it, so escaping from the test expression just skips to the next iteration. None of these scopes return values.
A subroutine call is a scope. It corresponds to the block scope of the body of the subroutine. Escaping from that scope returns from the subroutine. Values may be returned, depending on the context of the call.
A call to a format, via
write, is a scope. The main activity of a format is to output formatted text. Escaping early terminates the outputting activity from the format, but page end processing still occurs before
writereturns. No value is returned from the format.
The replacement part of a substitution (
s///) expression is evaluated in its own scope. The scope supplies the (scalar) substring to be inserted in place of what was matched.
- block eval
The block form of
eval, used to catch
dieexceptions, provides a scope, just like any other block. However, when the block returns normally,
$@is cleared to indicate that there was no exception.
evalis a type of expression, so the block commonly supplies a value.
- string eval
The string form of
eval, used to parse code at runtime, provides a scope in which the parsed code executes. In addition to parsing code at runtime, this has the exception handling behaviour of block eval. When the scope returns normally,
$@is cleared to indicate that there was no exception.
evalis a type of expression, so the scope commonly supplies a value.
If a file is parsed and executed, by
require, the entire file is a scope. Values may be returned, depending on the nature of the calling site.
These things are not scopes:
- conditional statement
The test expression of a conditional statement executes in the scope surrounding the conditional statement: there is no scope enclosing just the conditional statement. The blocks that execute conditionally, however, are each a scope, as normal for a code block.
- loop/conditional modifier
Statements involving postfix modifiers for looping or conditionals do not introduce any additional scopes. They are in this respect completely unlike the loop and conditional statements where the keyword comes first.
INACCESSIBLE STACK FRAMES
Using Perl's native control constructs, an
eval block (or one of its several equivalents) sets a limit on how far a non-local control transfer can travel. Except when exiting the entire process, the only way to non-locally transfer past the boundary of a single subroutine call is
eval block always stops the progress of a
die, and gives the catching code a choice about whether to set the
die going again through more stack frames. Some parts of Perl rely on the result of this: that with an
eval frame it is impossible for a non-local control transfer to pass one by.
As a result of this, it is not possible, in the general case, to use an escape continuation to cross over an
eval stack frame. These frames are effectively impervious to non-local returns. This module currently doesn't attempt to work around this limitation even in the cases where it would have a fair chance of success. When there is an
eval frame between the current code and the target of an escape continuation, the target is said to be "inaccessible". The continuation remains valid when this is the case, even though it will reject any attempt to actually transfer through it. Once the last intervening
eval frame has been exited, the target becomes accessible again, and the continuation can be used normally. The details of this may change in the future, though it is likely that there will always be some types of stack frame that are impervious.
The continuations implemented by this module are not first-class. That is, the existence of a continuation object does not keep its target stack frame in existence. A continuation has a limited period of validity, based on the treatment of its target, and so if a continuation object is retained long enough it will refer to a continuation that is no longer valid. Transfer through a continuation, and some other operations, are not permitted when the continuation is invalid. This implemenatation cannot always reliably detect that a continuation has become invalid, so the prohibited operations invoke undefined behaviour.
A continuation generally becomes invalid when its target stack frame is unwound. The simplest case of this is when the target block completes normal execution and returns normally. In that case, the continuation becomes invalid as soon as the block has completed execution and unwinding of the stack frame begins.
When a non-local control transfer occurs (such as
die, or use of an escape continuation from this module), continuations referencing stack frames higher than the target become invalid. They do this as soon as the control transfer is initiated, before any of the stack frames are actually unwound. However, if the non-local control transfer is the use of an escape continuation, that continuation itself remains valid during unwinding, until its target is unwound at the completion of the control transfer. Thus cleanup code executed during unwinding can itself perform non-local control transfers, provided that its target is at least as low as the target of the current unwinding.
If multiple continuations appear to target the same stack frame, such as the frame established by a subroutine call, they are always actually nested in some particular order. The earlier-established continuation is always the outer one. Effectively, the remainder of a block is nested inside the complete block. This corresponds to the way that (both lexically and dynamically) things later in a block can shadow things earlier in the block.
Nominally, local returns from stack frames don't have the complications of non-local control transfers. However, the way Perl performs them isn't quite as local as it should be, in part because of the facility for a block to set up several dynamic things in sequence. In continuation terminology, reaching the end of the block acts much like a non-local return to where the block was invoked, during which all of the block's cleanup code will run in sequence. Continuations for those intermediate scopes are all invalidated as soon as the interior of the block is complete, rather than (as would be the case with a truly local return) when the corresponding cleanup code runs. Also, the target continuation of a normal Perl return is invalidated when the return commences, so it is not valid to attempt a normal Perl return to the same target during unwinding.
These operators should be used through bareword function call syntax, as if they were functions. However, they cannot otherwise be called as functions in the normal manner. Attempting to take a reference to them will result in a code reference that does not have any of the behaviour described.
Reifies the current scope's escape continuation, returning it as a reference to an unblessed function. The function can be called through this reference in order to return from the current scope. The function can also be manually passed to the Scope::Escape::Continuation methods.
Reifies the current scope's escape continuation, returning it as a reference to a Scope::Escape::Continuation object. The methods of that class can be called through it. The object can also be called as a function in order to return from the current scope (the action of the go method).
This operator is to be preferred if you want to treat the continuation as an opaque object and want to use the Scope::Escape::Continuation methods. If you want to treat the continuation as a plain function, prefer "current_escape_function".
Andrew Main (Zefram) <email@example.com>
Copyright (C) 2010, 2011 Andrew Main (Zefram) <firstname.lastname@example.org>
This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.