[Python-ideas] A Continuations Compromise in Python

[Gerald Britton]

First off, I think that you misrepresent Python by your
characterization of it as a teaching language.  To be sure, many CS
programs _do_ teach Python as part of their curricula and use it to
teach core topics.  But Python is much more than that -- it is a
serious language for serious applications.  (If you're not sure about
this, just ask Google, which has built large parts of its
infrastructure on Python).

More importantly, while I appreciate your idea from a theoretical
perspective, I believe that your proposal would garner more attention
if you would provide some real-world examples.  If the expanded role
for _continue_ were already implemented, what real problems would be
easier to solve, or result in more elegant source code?  I, for one,
would like to see some "before" and "after" examples using the syntax
on typical applications.

On Sat, May 2, 2009 at 4:27 PM, John Graham <john.a.graham at gmail.com> wrote:
> Hi all
>
> It was suggested I post this idea to the python-ideas list.  I
> apologize in advance for its length, I tried to be thorough :)
>
> Recently there's been some debate over the Tail-Call-Optimization and
> its use in Python.  Some advocates coming from languages that support
> this optimization say that it makes recursive code far more efficient,
> and indeed allows some things to happen that could not otherwise occur
> without running out of stack space.
>
> The argument against, made most faithfully by the BDFL himself, is
> that looping behavior is in general easier to understand than
> recursion, and that implicitly optimizing code behind the scenes may
> confuse newcomers to the language.  This would especially affect stack
> traces.  Furthermore, Python is already an expressive language and
> there appear to be few use cases that truly demand TCO.
>
> Many of these arguments are strong, however, their strength can be
> used against them.  Python is a teaching language, and proudly so,
> which is one of the reasons to keep it clean and simple.  At the same
> time, recursion is an important part of Computer Science, software
> development and programming.  Indeed, many people's first recursive
> functions nowadays are written in Python!  Python is the perfect
> language to introduce these different kinds of constructs as the
> 'functional' paradigm is further absorbed by other mainstream
> languages in industry and academia.
>
> Other than it's potential to teach, Python's first class support of
> recursion, through an explicit mechanism to indicate 'continuation'
> style functions, is also undervalued in the use cases it more simply
> carries out compared to iterative implementations (never mind the
> performance benefit).  The Actor model and message passing, state
> machine logic, and green thread schedulers are all potential uses.
> Similarly, many multi-threaded/multi-processor programs simplify their
> logic when passing continuations.
>
> This proposal is to extend the use of the keyword 'continue' as a way
> to explicitly invoke continuation style function calls, and through
> that mechanism, tail-call optimized recursion.  The Zen of Python
> states that explicit is better than implicit, and this is the most
> major hang up with many proposed 'optimizations' that change system
> behavior.  In many TCO proposals, the compiler is responsible for
> identifying recursive calls that can be optimized.  In this proposal,
> continuations are achieved explicitly, and it is up to the programmer
> to ensure that they are proper tail calls, keeping all of the choice
> and power in the programmer's hands.  This is best shown with a few
> examples.
>
> Currently, a 'continuation' style function might be written like this:
>
> def func(continuationFunc, *args):
>        return continuationFunc(*args)
>
> The function is passed in as a first class object, then applied to
> some arguments and immidately returned.  This is an important point
> about true tail-calls – their values must immediately be returned.
> The proposed syntax extension would change the above function to the
> following similar function:
>
> def func(continuationFunc, *args):
>        continue continuationFunc(*args)
>
> In this case, instead of the 'return' keyword, the 'continue' keyword
> is used to indicate the programmer wishes to transfer full control to
> the continuationFunction.  Tail-call purity would be enforced, meaning
> that non-tail calls
>
> def func(continuationFunc, *args):
>        continue 1 + continuationFunc(*args)
>
> would either cause a syntax error or throw a runtime exception.
>
> This serves as a learning opportunity for those new to programming,
> just as many of Python's uniquely elegant implementations of
> programming constructs have, yet also provide a powerful and
> expressive way to solve certain kinds of recursive problems.
>
> The double use of the 'continue' keyword is no accident.  The current
> use of the 'continue' keyword makes a prime candidate for extension
> for a few reasons.  First there are theoretical reasons.  For one, the
> use of the English word 'continue' in the above constructs is
> intuitive and would not take much explaining to a newcomer as to what
> it means versus 'return'.
>
> More importantly, since it has been pointed out that many recursive
> structures can be implemented as imperative loops, it seems
> appropriate that the same keyword would mean equivalent/parallel
> things in the two styles.  Using continue inside a loop – bounce back
> up to the top of the loop and start with the next iteration, is
> synonymous with the proposed recursive use, to bounce back up the
> stack and immediately call the 'next' function.  Indeed, trampoline
> style schedulers would use the continue keyword in its current form to
> implement continuations!  These parallels between recursive and
> imperative loops help understanding the new proposed use.
>
> There are also some practical concerns.  'continue' as currently used
> is not that popular of a construct.  When one needs to 'continue', one
> is glad it's there, but due to Python's expressive generator
> expressions, for-each style loops and inner functions, many use cases
> for 'continue' become more elegantly expressed in other ways.  This
> means that, either way, a newcomers introduction to the uses of
> 'continue' aren't going to happen until he or she has a firm grasp of
> other parts of the language.  Moreover, there's no rule saying that a
> student has to learn about all the uses of a keyword upon being first
> introduced to the keyword.  A student would only inadvertently need to
> learn about continuation passing via 'continue' if they misunderstood
> the 'continue' statement while learning about loops, and somehow
> triggered the interpreter's wraith that they didn't define their
> continuation properly.  There's also no hard and fast rule that a
> student would learn about complex loop control before they'd be
> interested in continuation passing, so it works both ways.  'continue'
> also currently has a very well defined, strict syntax.  This is very
> important for any potential extensions, as it guarantees that this
> proposal is completely backwards compatible.  The uses of the keyword
> in the examples are currently syntax errors, meaning they can't be in
> any current running code.  The reuse of a keyword already in the
> language also guarantees there will be no naming collisions with older
> code.
>
> Using 'continue' in this way seems very 'Pythonic'.  It flows
> naturally from the meaning of the keywords, and the extended syntax is
> still rather simple – it must be a single callable.  There are no
> unique rules or corner cases.  It also mimics other return-control
> flow additions to the language, most notably the 'yield' construct,
> used for generators and co-routines.  Yield is overloaded, in a sense,
> to have two different yet similar purposes.  Overloading 'continue' in
> this way seems to naturally flow from the evolution of the language.
> Likewise, users are still free to intermix yields, returns and
> continues in functions and still have a very well defined, easy to
> understand behavior.
>
> Some have voiced concerns.  One instance is the appearance of a
> 'continue' in a try/catch/finally block.
>
>        try:
>                continue func(*args)
>        catch:
>                #error handle
>
> This appears to be an implementation problem, but only until you were
> to expand the same block into an equivalent, old-style 'return code'
> error handling scheme.
>
>        err = continue func(*args)
>        if(err):
>                #error handle
>        else:
>                return err
>
> In this form, it becomes apparent that this is an illegal use of
> continue, as 'func' is not a tail-call!  Checking the result of a
> tail-call and controlling logic via that doesn't make any sense, and
> as such, we can without remorse or regret say 'no tail calls in
> try/catch/finally' blocks.  Tail-calls are functional calls where
> there is absolutely no more processing to be done in the current stack
> frame.  'Clean up' logic or error handling in catch and finally blocks
> indicates there is more processing to be done, and as such, there
> simply is no well defined 'tail-call' in these circumstances.  They
> ought to remain normal stack returns.
>
> 'Continue' can certainly propagate exceptions as well as return
> values, but those would be caught at whichever stack level initiated
> the tail-call loop.
>
> Another concern is the fact that stack traces disappear with these
> types of constructs.  The counter argument is that the construct that
> they replace (in some instances) are loops, which themselves only have
> limited 'stack' trace information.  If you crash in a loop, you don't
> necessarily get the iteration of the loop you were on, or any
> information from previous iterations.  That is all lost.  Likewise, a
> naive implementation of the proposed 'continue' function would also
> lose stack trace information – however, it will do so in a way one can
> expect.  Just as if one were to implement a catch block that swallowed
> every exception and reported nothing, one should be familiar with the
> behavior changes in the stack that 'continue' would produce.
> Thankfully, unlike some behind the scenes optimization, a 'continue'
> keyword clearly shows where tail-calls are made in this proposal.  In
> a more advanced implementation, the clear use of 'continue' would
> allow the interpreter to annotate the stack trace, such that some
> information could easily be provided about current and historical
> program flow.
>
> There are some alternative designs for this problems that have been
> proposed.  SJBrown has proposed a similar syntax, using the two
> keywords 'continue as' instead of simply 'continue'.  This would
> further make clear to users unfamiliar with continuations that
> behavior is expected to be different here, and the English still flows
> elegantly.  It does, however, reuse the 'as' keyword in an unfamiliar
> way.  Other proposals followed the same logic, but introduced new
> keywords such as 'recurse' or 'tail'.  These avoid any potential
> ambiguity with the 'continue' keyword, but are not backwards
> compatible.  Furthermore, this proposal sees the actual English of the
> word 'continue' to be a strength in this case.
>
> Continuations and tail-calls are different, sometimes clearer, ways to
> implement advanced control flow.  They allow the programmer a lot of
> power in a small expressive and elegant package.  They are too
> frequently hidden behind the scenes, though, as the programmer relies
> on compiler identified 'optimization' spots.  Introducing explicit
> continuations to Python would allow students learning programming and
> advanced users both an expressive, yet very easy to learn and
> familiar, construct.  Extending the 'continue' keyword is the best
> candidate for this change, as its backwards compatible, parallels
> other similar extensions like yield, mimics the current use of the
> keyword, and is, in this author's opinion, 'Pythonic' ;)
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-- 
Gerald Britton