If you remember how the original toplevel code, parallelfib, was written, you might be worried now that continuationpassing style breaks the code, because now the root fibtask completes before the children are done, and the toplevel code used. This means we can do a lot more with the control flow of the program. Here are some thoughts concerning the continuation passing style in python. In the direct style, there is a hierarchy of functions. In the authors opinion, this approach gives the best of the two worlds. Tail function calls have a fairly direct translation into hardware. You can think of a continuation as what to do next in a program, and so continuation passing style means we make the continuation an explicit argument of a function. Sep 19, 2019 you just thread that k through there, and you have this style of programming is called continuation passing style.
Dont fully understand continuation yet, will have to dig later. Tailrecursive, continuation passing style the scala. Continuation passing style cps for short is a style of programming in which functions do not return values. So far, this is the essence of continuation passing style. What is continuationpassing style in functional programming. This paper examines the transformation of callbyneed. Continuationpassing style article about continuation. In this paper, we introduce continuation passing c cpc, a programming language for concurrent systems in which native and cooperative threads are unified and presented to the programmer as a single abstraction. We separate the code generation into phases that rewrite this representation into eversimpler forms. This idea of functions never returning, and callers passing their functions information about what needs to be done next, is a bizarre but powerful style of programming called continuation passing style.
Continuation passing style cps is a method of programming which allows for intricate manipulation of control flow while still maintaining functional purity. It begins by presenting a simple transformation of callbyneed. Of course, being able to take any function as an argument for another function, the language already allows to use such a style of programming when needed. In brief, a continuation is a functional representation of an expressions context, and we may view it as the rest of the computation. In a functional language this can be the rest of the program. The cpc compiler uses a compilation technique, based on the cps transform. These continuations are c functions and are passed on as extra arguments to other c functions. In april 1994 the three projects were combined and christened cilk. This book shows how continuationpassing style is used as an intermediate representation on which to perform optimisations and program transformations. If you understand about continuations, one way to explain what we did would be to say that we wrote our software in continuationpassing style. Keywords continuations, continuation passing style, monads, optimizing compilation, functional programming languages 1. Continuation passing s tyle in c and the cost of t ail call elimination with gcc. Lets take a break from type classes and take a brief look at one of the more esoteric concepts of functional programming.
Some code generators can cleverly eliminate the closures, but perhaps not if further transformations intervene. A correspondence between continuation passing style and static single assignment form. If this feature is generalised to an inline compiler then there might be a glimmer of hope. In proceedings of the third acm sigplan workshop on continuations cw01, pages 22, january 2001. There is another technique called continuation passing style abbreviate as cps to transform more complex. Continuationpassing, closurepassing style proceedings of. We implemented a continuationpassing style cps code generator for ml. You can try to use the term continuation monad to find some additional articles. Extended continuationpassing style,ecps delimited control operators shift. This lets me make sure i always assign a meaningful value, and dont miss the assignment in any of my branches or clauses. If youre new to continuationpassing style, i recommend my earlier post on. Ive read a little bit on continuationstack exchange network. This usage of a continuation helps collect the cross cutting concern and keeps it all on one place.
Continuation passing style often opens up an opportunity for scheduler bypass. Indeed, tail recursions will always transform into the same kind of lowlevel code as the loops by a compiler. That was itself written in continuationpassing style, taking an expression to. Nov 19, 20 lets take a break from type classes and take a brief look at one of the more esoteric concepts of functional programming. The top level function is a sort of master controller who calls one subroutine, and then another, deciding when to branch, when to loop, and generally coordinating the control flow explicitly. According to the wikipedia page on staticsingle assignment ssa, ssa is used by large and wellknown projects such as llvm, gcc, msvc, mono, dalvik, spidermonkey, and v8 while the page on projects using continuation passing style cps is a bit lacking in comparison. Callbyneed and continuationpassing style springerlink. This is a particularly common strategy in haskell, where it is easy to construct a continuation passing monad for example, the cont monad and contt monad transformer in the mtl library. A semantically clean language with continuations used as an intermediate language for scheme and the smlnj compiler. Cps continuation passing style is a particular way of defining and calling functions. This new style is called continuation passing style.
When it comes to kontinuations on the heap, i had to write the continuation k as a class fine but i had to use virtuality to let the compiler understand what i wanted if i use templates the compiler cannot understand that my program eventually terminates and for each call create a new type. Continuation passing style revisited, part one continuation passing style revisited part two. Find out information about continuation passing style. The idea described above is the basis of a compilation technique.
Plotkins value calculus is sound but incomplete for reasoning about eegr. Continuationpassingstylehas been used as a programming style as well as a compilation technique in functional programming languages for over 20 years. You could wrap up this concern other ways, notably with something like like aspectj. Programs can be automatically transformed from direct style to cps. Continuations can be used to implement a whole range of things.
The semantics of cpc is defined as a sourcetosource translation from cpc into plain c using a technique known as conversion into continuation passing style. The information about what to do next is called a continuation, which is passed from. Many modern compilers for higherorder functional lan. The method to convert the program into continuation passing style has been explained by matt might. The method is illustrated in a compiler for the programming language standard ml. Cps continuationpassing style is a particular way of defining and calling functions. Citeseerx document details isaac councill, lee giles, pradeep teregowda. Introduction compiling with continuations is out of fashion. Citeseerx continuation passing style in c and the cost. The concept of continuation passing style, or cps wikipedia has more info, however, seems to have a lot more to offer. Continuation passing style often abbreviated cps is an alternative way of writing programs in which every function called receives a continuation as an additional argument, and in which every function call is in tail position. We got around this by using lexical closures to simulate subroutinelike behavior. The cpc compiler uses a compilation technique, based on the cps transform, that yields efficient code and an extremely lightweight representation for contexts.
Recently it has been discovered that cps resolves a longstanding structural problem in the composition of software reliability. In cps, all the function calls are actually jumps to continuations that never return so you asymptotically better not waste stack space with them. Why is staticsingle assignment preferred over continuation. Scheduler bypass is an optimization where you directly specify the next task to run. It translates slightly flavored javascript syntax into standard javascript, so it can be also called a translator. Chicken scheme compiler, a scheme to c compiler that uses continuationpassing style for translating scheme procedures into c functions while using the cstack as the nursery for the generational garbage collector. Now, this is not a pleasant style to write code in, but your compiler can turn your straightline, imperative, do this, do this, do this, call that, nested function calls, you can do all that. Continuations can be used to compile most programming languages. For this reason our compiler uses the continuation passing style cps conversion 2 to eliminate nontail function.
Cs 6110 lecture continuationpassing style 20 february 20 lecturer. You can write continuation passing programs in python, or in any language that. Pdf continuation passing style in c and the cost of tail. Continuationpassing style in javascript matt might.
Im not yet done with that, but after hours of racking my brains, its finally on a good way as it seems. There is another technique called continuationpassing style abbreviate as cps to transform more complex. We implemented a continuation passing style cps code generator for ml. Handwaving about control flow continuation passing style revisited part three. Musings about coroutines continuation passing style revisited part four. These c functions never reach the return statement. You just thread that k through there, and you have this style of programming is called continuation passing style. Haskellcontinuation passing style wikibooks, open books. Continuationpassing style cps is guiles principal intermediate language, bridging the gap between languages for people and languages for machines. Functional and logic compilers often use cps as an intermediate representation where a compiler for an imperative or procedural programming language. Continuationpassing style cps originated as a style of programming in the 1970s.
Continuation passing style chaotic good programming. Cs 6110 lecture continuationpassing style 20 february. Citeseerx continuation passing style in c and the cost of. The solution to this situation is called continuation passing style cps, and is very similar to the accumulator method. It will be essential reading for compiler writers in both industry and academe, as well as for students and researchers in programming language theory. Treeil is structured in the sense that its representation is. Continuationpassing style cs 6520, spring 2002 1 continuations and accumulators in our exploration of machines for iswim, we saw how to explicitly track the current continuation for a computation chapter 8 of the notes. So report the authors of two classic papers on continuationpassing style in recent retrospectives. In functional programming, continuationpassing style cps is a style of programming in which.
I have recently been struggling to implement transformation of common lisp source code to continuation passing style cps in my owlisp compiler. One of the problems with using web pages as a ui is the inherent statelessness of web sessions. Partially cpsconverting a program makes for natural node. Continuation passing style article about continuation. Andrew myers continuationpassing style cps cps semantics cps conversion 1 the limitations of direct translation so far we have been building translations that preserve control.
To find a complete extension, we define a new, compactif. And compilers are able to aggressively optimize programs written in continuation passing style because everything the compiler needs to know about a function is available in the continuation. Continuationpassing style cps is guiles principal intermediate language, bridging. This book shows how continuation passing style is used as an intermediate representation on which to perform optimisations and program transformations. In my view, these sorts of questions need an answer in two parts. You can macroexpress normal function calls into cps by. Reasoning about programs in continuationpassing style. Then, perhaps with continuation passing style, the compiler creates efficient code but then that is not necessarily so due to the style, since theres no reason a function sequencing or nesting style wouldnt produce the same background for creating efficient machine code. Continuationpassing style continuationpassing style, aka cps, is a paradigm for expressing programs in terms of primitives that explicitly build, pass around and invoke continuations.
Your task is to experiment with continuation passing style and to write at least one function, anything you like, in continuation passing style. Our cps language is represented as an ml datatype in which all functions are named and most kinds of illformed expressions are impossible. For example, at the end of the continuation passing example in the previous section, method execute spawns task a and returns. The reader of appels inspirational book 1 may be thinking just use continuationpassing style cps. If you can write a compiler that turns your favourite language into cps, and you have.
In continuation passing style, callcc becomes a simple function that can be written with lambda. While this provides yet another argument in favour of. Nontail function calls can be translated to tail function calls which pass an additional continuation parameter in the message. The issue with doing it with something like aspectj is that it is much less accessible than the continuation passing style.
Continuationpassingstyle has been used as a programming style as well as a compilation technique in functional programming languages for over 20 years. Chicken scheme compiler, a scheme to c compiler that uses continuation passing style for translating scheme procedures into c functions while using the cstack as the nursery for the generational garbage collector. In 2002, then, cps would appear to be a lesson aban. In the continuation passing style, though, there is. The current cpc compiler is mature enough to write substantial programs such. Continuation passing style in c and the cost of tailcall elimination with gcc. The use of cps as a compiler representation has a long his. If you can write a compiler that turns your favourite language into cps, and you have an orchestrator that knows how to dispatch the next continuation without consuming stack, then as weve seen, you have the all the building blocks you need to add any control flow. Apr 03, 2018 and compilers are able to aggressively optimize programs written in continuation passing style because everything the compiler needs to know about a function is available in the continuation. Continuationpassing style, or cps more shortly, is a style of programming sometimes. In a compiler based on continuationpassing style cps, this frame is the closure of a continuation function steele 1978.
The name cilk is not an acronym, but an allusion to nice threads and the c programming language. Pcmthreadedc a cbased package for scheduling continuationpassingstyle threads on the cm5. In this chapter, we are going to consider how that plays out in haskell and, in particular, how cps can be expressed with a monad. From this, an informal derivation is carried out, resulting in a translation from. Continuation passing style in c and the cost of tailcall. Tree intermediate language treeil is a structured intermediate language that is close in expressive power to scheme. Continuationpassing, closurepassing style proceedings. Roughly, the continuation re ects the control stack in a real machine, including virtual machines such as the jvm. Oct 26, 2010 continuation passing style revisited part four.