Go to, let us go down, and there confound their language, that they may not understand one another's speech.I've been asked about Kernel's guarded continuations. In writing that section of the Kernel Report, I knew it wasn't as lucid as one might like, but getting it all down while I had it clearly in mind was something of an achievement in itself. I'm not sure I've ever seen a really lucid explanation of continuations; explaining them should probably be considered a major challenge in its own right, above and beyond figuring out how the mechanism should work. It's high time I tried to do better, though; however many tries it takes to get the explanation to come clear, I won't get there if my number of tries never increases.
— Genesis 11:7 (King James Version)
On my Kernel web page I describe guarded continuations as 'Exception handling "done right"'. For an ultra-short tag line, that may be about as good as any; but guarded continuations are more a simultaneous blend-and-improvement on Scheme continuations and Java exception handling, rather than a supposedly-optimal exception facility. We may still be several generations of exception facilities away from getting exceptions right; but as may be, I think guarded continuations have something to add to the discussion.
It's also worth saying up front that guarded continuations are lower-level than a good exception facility would be. Kernel tries for very simple low-level facilities plus the ability to build higher-level abstractions from them. It's an open, and fascinating, question how well Kernel's abstraction facilities will actually apply to exception handling — and I hope to learn a lot from that question. (I expect shortcomings of Kernel's abstractive facilities to be not about the facilities' power (which I maintain is extraordinary) but about the directions in which that power can be applied. When I figure out what the missing directions are, I mean to go that way.)
Stacks and trees
Classically, programming languages deal with function calls using a stack.  There are two kinds of stuff on the stack: local variables (data), and return addresses (control). The local data for a function call produces a single set of local variables; but the algorithm for that same call may produce many different return addresses, one for each point in the algorithm where it, in turn, calls a function. In principle, one might have two stacks, one for data and one for control; but classically, all local variables and all return addresses are use-once, to be discarded in the same order they were introduced, so it may seem natural to store both on a single stack (with resultant security problems, malware accessing data as control and vice versa).
First-class (lexically scoped) functions require local data to be stored, in general, as a tree rather than a stack. When a first-class function is returned as the result of a function call (in vintage parlance, as an upward funarg), lexical scoping expects that function to have access to —in general— the entire column of variables that were on the stack when the function was created, even though some of the function calls that created those variables have since ended. So data that used to be popped off the stack must now, potentially, persist. A set of local variables, which used to be the data part of a stack frame, is now an environment, with a fixed set of ancestors (variables further up the column) and potentially any number of children (one for each call to any function defined in that environment). (In Kernel, btw, environments are first-class objects; there was a lively LtU discussion on first-class environments, here.)
First-class continuations do to the control stack what first-class functions did to the data stack: they turn it into a tree. (First-class continuations do this to the data stack too, but I'm not aware of any language that has first-class continuations without first-class functions.) Just as first-class functions required entire columns of local variables to persist after the ends of function calls that created them, first-class continuations require entire columns of "return addresses" to persist after the ends of function calls that created them.
In visualizing the tree of continuations, the actual algorithms of the program hover in the areas beneath the nodes of the tree (possibly, multiple algorithms under a single node). Each child continuation is a socket waiting for a value to be plugged into it, to be chewed on by the algorithm above that child — which may, eventually, terminate and plug a value into its parent continuation. Primitive operations hover beneath the leaves of the tree. Kernel pretty much only needs one non-primitive operation: combination evaluation, which spins off a subtree to evaluate the operator, spins off subtrees as appropriate to evaluate operands, and finally hands the situation over to another algorithm to make the operative call (using for the operative call its parent continuation, in properly tail-recursive fashion).
When a function f calls another function g, f gives up some of its ability to regulate the future evolution of the computation. The programmer tends to assume that not all such power has been lost — that (1) the call to g won't terminate without control passing back through the point at which f called it, and (2) that particular call to g won't return through that point more than once. In the presence of first-class continuations, though, even these weak assumptions needn't hold. Once f calls g, the dynamic extent of that call might be exited, and/or later reentered any number of times, in ways that bypass f entirely. Perhaps f opens a file, calls g, and then closes the file before relinquishing control to whoever called f; if g terminates bypassing f, the file might not be closed outside the call to f, and if g is reentered bypassing f, the file might not be open inside the call to g.
Scheme addresses this sort of problem by means of dynamic-wind, a device that causes specified actions to be performed whenever a continuation-call tries to enter or leave the dynamic extent of the call to g. In R5RS Scheme (the version Kernel primarily diverged from), there were a couple of problems with dynamic-wind.
- The R5RS was explicitly baffled by what would happen if continuations were used to pass into or out of the dynamic extents of the calls to before and after — which suggested to me that the whole thing was something of a kludge. The correct behavior in such cases ought to follow naturally from the conceptual framework, and if it didn't follow naturally one had to wonder if the conceptual framework was altogether coherent.
- Mostly obviously and trivially, it only allows exiting a dynamic extent, not reentering one.
- An intercepting catch has access to the value being thrown (unlike the after of dynamic-wind).
- A catch selectively intercepts throws, based on the "target" (one might even say, "destination") of the throw. Whereas Lisp cleanly separates the target of an abnormal transfer of control from the value being transferred —the continuation called, versus the value passed to it— Java entangles them in a single thrown object whose type is the "target".
- Once a Java exception is caught, that's the end of the exception unless it is explicitly rethrown — in contrast to the after of dynamic-wind, which ordinarily does not disrupt the abnormal transfer of control during which it is called.
Kernel's guarded continuations drew on both these models for inspiration. An applicative guard-continuation allows the construction of a continuation with entry- and exit-guard clauses. Exit-guard clauses selectively intercept continuation-calls from inside to outside the guarded dynamic extent (the branch of the continuation tree descending from the guarded continuation), and are therefore analogous to Java catch clauses, with some adjustments. Entry-guard clauses are symmetrically designed to selectively intercept continuation-calls from outside to inside the guarded extent.
Each guard clause has a selector continuation. Keep in mind that a continuation-call has two ends: it goes from one point in the continuation-tree to another, and in this case we know that one end of the call is inside the guarded extent, while the other end is outside. A continuation-call matches the selector of a clause just if the outside end of the call lies within the tree branch descending from the selector. For an exit-guard clause this means that the called continuation must be in the selector's branch of the tree. For an entry-guard, the starting point of the continuation-call must be within the selector's branch of the tree.
Because entry guards are selected based on the source of the continuation-call, a list of interceptors selected when the continuation-call is initiated will be different, in general, from what one would get by stopping at some intermediate interceptor and then "rethrowing" the call (in Java catch style). This suggests that normal termination of an intercepting guard should continue along the path of the original continuation-call, reserving explicit diversion for when the interceptor doesn't want to follow the original path. Secondarily, although we do want it to be convenient (as well as possible) for an interceptor to continue along the path of the original continuation-call, we don't really want to give the interceptor first-class continuations for either end of the call (source or destination). We may have built some abstract device on top of the continuation primitives, and as a result, the source/destination continuations may be internal details that really oughtn't be casually spread about.
Therefore, normal termination of an intercepting guard continues along the path of the continuation-call, and the interceptor when called is provided with an explicit continuation for diverting control to just outside the guarded extent (Java-style).
For versatile guard selection, each guarded gateway (entry or exit for a dynamic extent) has a list of guard clauses. When a continuation-call goes through the gate (entering or exiting), the selectors of these clauses are checked, one at a time, until one is found that matches the call, or until there are no more clauses so that no interception occurs. No more than one clause is selected. A root continuation is provided, which can be used as a selector that matches all possible calls. Using the root continuation, plus the ability for an interceptor to continue along the continuation-call path, one can (for example) arrange that a clause will be selected except when the call matches some narrower selector (for example, the error continuation, which is the ancestor of all evaluation-aborting error handlers). One provides a clause that catches errors and causes them to continue along their path, followed by a clause that catches everything and does whatever it is one wants done only in non-error situations.
When a continuation-call is made, a list of intercepting guard clauses can be immediately constructed, since continuation guards are fixed at the time the guarded continuation is constructed. The list doesn't have to be constructed then, of course, exactly because all the relevant parts of the continuation tree are immutable; but if it isn't constructed immediately, one will have to store the information needed to select the clauses later — which includes the source of the call as well as its destination (destination to select exit guards, source to select entry guards).
The Kernel Report carefully works out subtleties of the device, such that when a guard intercepts an entry/exit to dynamic extent d, and then itself makes a continuation-call, that call does not select any guard on d.
The traditional Scheme device for acquiring a first-class continuation object is call/cc, which calls a procedure and passes to that procedure the continuation to which that call would normally return. Frankly, this was always a very clumsy way to work with continuations; one might almost suspect it was devised as an "esoteric programming language" feature, akin to INTERCAL's COME FROM statement. With guarded continuations, though, dependence on call/cc finally becomes untenable. If I'm working in Java, and I want to create a new class of exception, I do that by adding a subtype to the tree of exception types; to do the same thing in Kernel, it seems I want to add a new continuation to the tree branch of continuations descending from the error continuation. If I had to use call/cc to acquire continuations, I could only add a new descendant of the error continuation if I were actually in in the dynamic extent of the error continuation — so, to define a new "type" of error during normal computation, I'd have to first "throw" an error exception, passing through all the exit- and entry-guards to get to the point in the tree where I want to add the new continuation. What's wanted is ability to add a child continuation to an existing continuation without having to be inside the dynamic extent of the parent. Kernel provides just that ability, with extend-continuation (to add some algorithm between child and parent) and guard-continuation (to add guards between child and parent). (Interestingly, call/cc still has to be provided as a primitive, since it can't be derived from the other primitives.)
An illustrative example
Alas, no. Source code for continuation guarding is generally bulky and hard to read, and attempting to illuminate the basic concepts of continuation guarding that way would more likely just create an unhelpful mess. For me, at least, it's all best understood by visualizing the tree of nested dynamic scopes, whereas the source code only directly reveals a tree of nested static syntax; the dynamic tree then has to be deduced, and the two trees don't correspond as closely as one might think.
How best to arrange for a clear illustration, I'll have to give some more thought. Meanwhile, imagine there was an illustration here that made it all crystal clear to you.
I've also been asked to discuss guarded continuations versus delimited continuations. <cough> That's a rather large topic. I'd probably have to start by explaining why I think "delimited continuations" is a misnomer — by my reckoning, delimited continuations aren't continuations at all. They're functions, and functions are a completely different sort of creature than continuations. Continuations only have one side (input), which is why it's simply meaningless to talk about composing them. (Can you see I'm working up to a major rant, here?)
So I'm going to end this post here —it's already the longest post I've yet written on this blog— and hope to put together something on delimited continuations later. Perhaps that post will also help to further clarify guarded continuations, but we'll see. It's yet another item for my collection of in-development blog posts (along with guarded environments, another interesting topic this has reminded me of).