# Continuations and Delimited Control

## Introduction to programming with shift and reset

The tutorial on delimited continuations was given together with Kenichi Asai (Ochanomizu University, Japan) in the evening before the Continuation Workshop 2011.

The concept of continuations arises naturally in programming: a conditional branch selects a continuation from the two possible futures; raising an exception discards a part of the continuation; a tail-call or goto continues with the continuation. Although continuations are implicitly manipulated in every language, manipulating them explicitly as first-class objects is rarely used because of the perceived difficulty.

This tutorial aims to give a gentle introduction to continuations and a taste of programming with first-class delimited continuations using the control operators shift and reset. Assuming no prior knowledge on continuations, the tutorial helps participants write simple co-routines and non-deterministic searches. The tutorial should make it easier to understand and appreciate the talks at CW 2011.

We assume basic familiarity with functional programming languages, such as OCaml, Standard ML, Scheme, and Haskell. No prior knowledge of continuations is needed. Participants are encouraged to bring their laptops and program along.

Version
The current version is September 27, 2011
References
CW2011 Tutorial Session. September 23, 2011
<http://logic.cs.tsukuba.ac.jp/cw2011/tutorial.html>
<http://pllab.is.ocha.ac.jp/~asai/cw2011tutorial/>

Tutorial notes for OchaCaml and Haskell
<http://pllab.is.ocha.ac.jp/~asai/cw2011tutorial/main-e.pdf>

ContExample.hs [4K]
A sample shift/reset code in Haskell, in the Cont monad -- the monad for delimited control

ContTutorial.hs [9K]
The complete code for the Haskell portion of the tutorial

## Delimited control and breadth-first, depth-first, and iterative deepening search

This tutorial-like Haskell code illustrates the application of delimited control for non-deterministic search. We apply different search strategies to the same non-deterministic program without re-writing it. A non-deterministic computation is reified into a lazy search tree, which can then be examined in different ways. We write non-deterministic search strategies as standard depth-first, breadth-first, etc., tree traversals.

The search tree is the ordinary tree data type, with branches constructed on demand. The tree is potentially infinite, as is the case in the example below.

    data SearchTree a = Leaf a | Node [() -> SearchTree a]

We implement three tree traversals, which collect the values from leaf nodes into a list:
    dfs, bfs, iter_deepening :: SearchTree a -> [a]

(Actually we use several versions of breadth-first search, optimized to a different extent.)

Using the Cont monad from the standard monad transformer library and its operations shift and reset, we implement two primitives: non-deterministically choosing a value from a finite list, and reifying a computation into a SearchTree:

    choose :: [a] -> Cont (SearchTree w) a
reify  :: Cont (SearchTree a) a -> SearchTree a

Other non-deterministic operations -- failure, mplus (to join two computations), choose' (to choose from a potentially infinite list) -- are all written in terms of choose.

The running example is a simple version of a real inductive-programming problem: given a sequence of input-output pairs [(Int,Int)], find an Int->Int function with that input-output behavior. The functions to search among are represented by the data structure:

    data Exp =
K Int                                 -- constant function
| X                                   -- identity
| Exp :+ Exp                          -- \x -> f x + g x
| Exp :* Exp                          -- \x -> f x * g x

The solution is the familiar generate-and-test
    induct io = reify $do exp <- an_exp if all (\ (i,o) -> eval exp i == o) io then return exp else failure  where an_exp generates a sample function representation, and the if-expression tests if evaluating it on given inputs gives the desired outputs. The generator of Exp expressions is  an_exp = (fmap K$ choose numbers)   mplus
(return X)                  mplus
(liftM2 (:+) an_exp an_exp) mplus
(liftM2 (:*) an_exp an_exp)
where numbers = [-2..2]

Depth-first search cannot be used for this problem since the search tree is infinite. Breadth-first and iterative deepening are both complete strategies and both find the answer if it exists. For example, for the sequence [(0,1), (1,1), (2,3)] of input-output pairs, we find K 1 :+ (X :* (K (-1) :+ X)) (which corresponds to the function 1 + x*(x-1)), which indeed has the given behavior. Benchmarking on a slightly bigger problem [(0,1), (1,1), (2,3), (-1,3)] shows that the optimized breadth-first search takes 303MB whereas iterative deepening takes 64MB of memory (and roughly the same time). Although toy, this inductive programming problem is not simple. For input-output pairs [(0,1), (1,3), (-1,3), (2,15)], breadth-first search quickly allocates 8GB and is killed by the kernel. Iterative deepening allocates at much slower pace, but still reaches 8GB and dies as well.
Version
The current version is February, 2022
References
Searches.hs [14K]
The complete Haskell code with many comments, explanations and tests. The comments tell why the search tree is defined as it is, with a thunk.

Embedded probabilistic programming
The paper explains the reification of non-deterministic programs as lazy search trees. We use the same technique here, only in Haskell rather than OCaml, and without probabilities.

Preventing memoization in (AI) search problems
The explanation of the trick to prevent unwelcome implicit memoizations

Joachim Breitner: dup -- Explicit un-sharing in Haskell
July 2012. <https://arxiv.org/abs/1207.2017>
An extensive discussion of unwanted memoization and ways to prevent it

## How to remove a dynamic prompt: static and dynamic delimited continuation operators are equally expressible

This technical report shows that the delimited continuation operators shift, control, shift0, etc. are all macro-expressible in terms of each other. Furthermore, the operators shift, control, control0, shift0 are the members of a single parameterized family, and the standard CPS is sufficient to express their denotational semantics.

The report formally proves that control implemented via shift indeed has its standard reduction semantics.

The report presents the simplest known Scheme implementations of control, shift0 and control0 (similar to cupto). The method in the report lets us design 700 more delimited control operators, to split and compose stack fragments as one thinks fit.

References
impromptu-shift-tr.pdf [136K]
<http://www.cs.indiana.edu/cgi-bin/techreports/TRNNN.cgi?trnum=TR611>
Technical Report TR611, Department of Computer Science, Indiana University, 2005

delim-cont.scm [10K]
Scheme code with the simplest implementation of control, shift0, control0 in terms of shift/reset. The code includes a large set of test cases.

Lambda the Ultimate'' discussion thread, esp. on the meaning of delimited contexts
<http://lambda-the-ultimate.org/node/view/606>

impromptu-shift-tr.scm [61K]
The master SXML file of the report

## Answer-Type Modification without tears: Prompt-passing style translation for typed delimited-control operators

The salient feature of delimited-control operators is their ability to modify answer types during computation. The feature, answer-type modification (ATM for short), allows one to express various interesting programs such as typed printf compactly and nicely, while it makes it difficult to embed these operators in standard functional languages. In this paper, we present a typed translation of delimited-control operators shift and reset with ATM into a familiar language with multi-prompt shift and reset without ATM, which lets us use ATM in standard languages without modifying the type system. Our translation generalizes Kiselyov's direct-style implementation of typed printf, which uses two prompts to emulate the modification of answer types, and passes them during computation. We prove that our translation preserves typing. As the naive prompt-passing style translation generates and passes many prompts even for pure terms, we show an optimized translation that generate prompts only when needed, which is also type-preserving. Finally, we give an implementation in the tagless-final style which respects typing by construction.

Joint work with Ikuo Kobori and Yukiyoshi Kameyama.

Version
The current version is June 2016
References
Electronic Proceedings in Theoretical Computer Science EPTCS 212 (Post-Proceedings of the Workshop on Continuations 2015) June 20, 2016, pp. 36-52 doi:10.4204/EPTCS.212.3

## Persistent delimited continuations for CGI programming with nested transactions

We present a simple CGI framework for web programming with nested transactions. The framework uses the unmodified OCaml system and an arbitrary, unmodified web server (e.g., Apache). The library makes writing web applications (CGI scripts) as straightforward as writing interactive console applications using read and printf. We write the scripts in the natural question-answer, storytelling style, with the full use of lexical scope, exceptions, mutable data and other imperative features (if necessary). The scripts can even be compiled and run as interactive console applications. With a different implementation of basic primitives for reading and writing, the console programs become CGI scripts.

Our library depends on the delimcc library of persistent delimited continuations. The captured delimited continuations can be stored on disk, to be later loaded and resumed in a different process. Alternatively, serialized captured continuations can be inserted as an encoded string into a hidden field of the response web form. The use of continuations lets us avoid iterations, relying instead on the Back button.' Delimited continuations naturally support thread-local' scope and are quite compact to serialize. The library works with the unmodified OCaml system as it is.

Delimited continuations help us implement nested transactions. The simple blog application demonstrates that a user may repeatedly go back-and-forth between editing and previewing their blog post, perhaps in several windows. The finished post can be submitted only once.

Version
The current version is 1.7, April 2008
References
Fest2008-talk.pdf [204K]
Fest2008-talk-notes.pdf [244K]
The demonstration of the library at the Continuation Fest 2008. The extended version of the talk, Clicking on Delimited Continuations'', was presented at FLOLAC in July 2008. The extended version includes a detailed introduction to delimited continuations.

caml-web.tar.gz [16K]
The source code for the library of delimited-continuation--based CGI programming with form validation and nested transactions. The library includes the complete code for the Continuation Fest demos.

## Simply typed lambda-calculus with a typed-prompt delimited control is not strongly normalizing

Simply typed lambda-calculus has strong normalization property: any sequence of reductions of any term terminates. If we add delimited control operators with typed prompts (e.g., cupto), the strong normalization property no longer holds. A single typed prompt already leads to non-termination. The following example has been designed by Chung-chieh Shan, from the example of non-termination of simply typed lambda-calculus with dynamic binding. It uses the OCaml delimited control library. The function loop is essentially fun () -> Omega: its inferred type is unit -> 'a; consequently, the evaluation of loop () loops forever.
    let loop () =
let p = new_prompt () in
let delta () = shift p (fun f v -> f v v) () in
push_prompt p (fun () -> let r = delta () in fun v -> r) delta


Chung-chieh Shan also offered the explanation: the answer type being an arrow type hides a recursive type. In other words, the delta's type unit -> 'a hides the answer type and the fact the function is impure.

Olivier Danvy has kindly pointed out the similar non-terminating example that he and Andrzej Filinski designed in 1998 using their version of shift implemented in SML/NJ. Their example too relied on the answer type being an arrow type.

Version
The current version is September 30, 2006
References
Carl A. Gunter, Didier R'emy and Jon G. Riecke: A Generalization of Exceptions and Control in ML-Like Languages
Proc. Functional Programming Languages and Computer Architecture Conf., June 26-28, 1995, pp. 12-23.
The paper that introduced cupto, the first delimited control operator with an explicitly typed prompt

Delimited Dynamic Binding
The reformulation in terms of shift and simply typed lambda-calculus

General recursive types via delimited continuations
A differently-formulated proof: representing general recursive types

## A substructural type system for delimited continuations

We propose type systems that abstractly interpret small-step rather than big-step operational semantics. We treat an expression or evaluation context as a structure in a linear logic with hypothetical reasoning. Evaluation order is not only regulated by familiar focusing rules in the operational semantics, but also expressed by structural rules in the type system, so the types track control flow more closely. Binding and evaluation contexts are related, but the latter are linear.

We use these ideas to build a type system for delimited continuations. It lets control operators change the answer type or act beyond the nearest dynamically-enclosing delimiter, yet needs no extra fields in judgments and arrow types to record answer types. The typing derivation of a direct-style program desugars it into continuation-passing style.

Joint work with Chung-chieh Shan.

Version
The current version is 1.1, June 2007
References
Type checking as small-step abstract evaluation
Detailed discussion of the two main slogans of the paper:
• Types are abstract expressions (Cousot)
• The colon is a turnstile (Lambek)

delim-control-logic.pdf [250K]
The extended (with Appendices) version of the paper published in Proc. of Int. Conf. on Typed Lambda Calculi and Applications (TLCA), Paris, June 26-28, 2007 -- LNCS volume 4583.

small-step-typechecking.tar.gz [10K]
Commented Twelf code accompanying the paper
The code implements type checking -- of simply-typed lambda-calculus for warm-up, and of the main lambda-xi0 calculus -- and contains numerous tests and sample derivations.

## Call-by-name typed shift/reset calculus

We present a Church-style call-by-name lambda-calculus with delimited control operators shift/reset and first-class contexts. In addition to the regular lambda-abstractions -- permitting substitutions of general, even effectful terms -- the calculus also supports strict lambda-abstractions. The latter can only be applied to values. The demand for values exerted by reset and strict functions determines the evaluation order. The calculus most closely corresponds to the familiar call-by-value shift/reset calculi and embeds the latter with the help of strict functions.

The calculus is typed, assigning types both to terms and to contexts. Types abstractly interpret operational semantics, and thus concisely describe all the effects that could occur in the evaluation of a term. Pure types are given to the terms whose evaluation incurs no effect, i.e., includes no shift-transitions, in any context and in any environment binding terms' free variables, if any. A term whose evaluation may include shift-transitions has an effectful type, which describes the eventual answer-type of the term along with the delimited context required for the evaluation of the term. Control operators may change the answer type of their context.

References
cbn-xi-calc.elf [19K]
Twelf code that implements the dynamic semantics (the eval* relation) and the type inference (the teval relation). The teval relation is deterministic and terminating, thus constructively proving that the type system for our Church-style calculus is decidable. The code includes a large number of examples of evaluating terms and determining their types.

gengo-side-effects-cbn.pdf [161K]
Call-by-name linguistic side effects
ESSLLI 2008 Workshop on Symmetric calculi and Ludics for the semantic interpretation. 4-7 August, 2008. Hamburg, Germany.

Compilation by evaluation as syntax-semantics interface
Linguistics turns out to offer the first interesting application of the typed call-by-name shift/reset. The paper develops the calculus in several steps, presenting the syntax and the dynamic semantics of the final calculus in Figure 3 and the type system in Figures 4 and 5. The paper details several sample reductions and type reconstructions, and discusses the related work.

A substructural type system for delimited continuations
That TLCA 2007 paper introduced the abstract interpretation technique for reconstructing the effect type of a term in a calculus of delimited control. The technique progressively reduces a term to its abstract form, i.e., the type. The TCLA paper used a call-by-value calculus with a so-called dynamic control operator, shift0. Here we apply the technique to the call-by-name calculus with the static control operator shift.

## Fixpoint combinator from typed prompt/control

In the recent paper Typed Dynamic Control Operators for Delimited Continuations' Kameyama and Yonezawa exhibited a divergent term in their polymorphically typed calculus for prompt/control. Hence the latter calculus, in contrast to Asai and Kameyama's polymorphically typed shift/reset calculus, is not strongly normalizing. Unlike the untyped case, typed control is not macro-expressible in terms of shift. Kameyama and Yonezawa conjectured that the (typed) fixpoint operator is expressible in their calculus too. The conjecture is correct: Here is the derivation of the fixpoint combinator, using the notation of their paper. The combinator is not fully polymorphically typed however: the result type must be populated.

Let f be a pure function of the type a -> a and d be a value of the type a (in the paper, d is written as a black dot). As in the paper, we write # for prompt. The expression

    #( control k.(f #(k d; k d)) ; control k.(f #(k d; k d)) )

appears to be well-typed. It reduces to
    #(f #(k d; k d))  where k u = u; control k.(f #(k d; k d))
then
#(f #(f #(k d; k d)))
eventually to
#(f #(f #(f ..... )))


Since we are in a call-by-value language, the above result is not terribly useful, but it is a good start. We only need an eta-expansion: Suppose f is of the type (a->b) -> (a->b). Let d be any value of the type a->b: this is the witness that the return type is populated. We build the term

    FX = #( control k.(f (\x . #(k d; k d) x)) ; control k.(f (\x . #(k d; k d) x)) )

that is well-typed and expands as
    #(f (\x . #(k d; k d) x) ) where k u = u; control k.(f (\x . #(k d; k d) x))
and then
f (\x . #(k d; k d) x)

noticing that k d is control k.(f (\x . #(k d; k d) x)) we get f (\x . FX x)

Thus we obtain that FX x is equal to f (\x . FX x) x, which means FX is the call-by-value fixpoint of f.

Without access to the implementation of Kameyama and Yonezawa's calculus, we can test this expression using the cupto-derived control. The latter is implemented in OCaml. We cannot test the typing of our fix, since the type system of cupto is too coarse. We can test the dynamic behavior however. To avoid passing the witness that the result type is populated, we set the result type to be a->a, which is obviously populated, by the identity function.

Version
The current version is 1.1, Oct 24, 2007
References
Yukiyoshi Kameyama and Takuo Yonezawa: Typed Dynamic Control Operators for Delimited Continuations. FLOPS 2008.

Kenichi Asai and Yukiyoshi Kameyama: Polymorphic Delimited Continuations
Proc. Fifth Asian Symposium on Programming Languages and Systems (APLAS 2007), LNCS
<http://logic.cs.tsukuba.ac.jp/~kam/paper/aplas07.pdf>

## General recursive types via delimited continuations

A general recursive type is usually defined (see Kameyama and Yonezawa) as \mu X. F[X] where X may appear negatively (i.e., contravariantly) in F[X]. If X appears only positively (as in the type of integer lists, \mu X. (1 + Int * X))), the resulting type is often called inductive.

A general recursive type, e.g., \mu X. X->Int->Int can be characterized by the following signature:

    module type RecType = sig
type t     (* abstract *)
val wrap   : (t->int->int) -> t
val unwrap : t -> (t->int->int)
end

provided that (unwrap . wrap) is the identity. If we have an implementation of this signature, we can transcribe a term such as \x. x x from the untyped lambda-calculus to the typed one.

ML supports one implementation of RecType`, using iso-recursive (data)types. However, there is another implementation, using ML exceptions. Since exceptions are a particular case of delimited control, we obtain another proof that simply typed lambda-calculus with a cupto-like delimited control is not strongly normalizing.

Version
The current version is 1.1, Oct 30, 2007; 1.2, Oct 2011
References
delimcc-rectype.ml [2K]
Complete commented OCaml code

## Call-by-need via delimited continuations

Call-by-need, or lazy, evaluation is call-by-name evaluation with the memoization of the result. A lazy expression is not evaluated until its result is needed. At that point, the expression is evaluated and the result is memoized. Thus a lazy expression is evaluated at most once. Lazy expressions may nest: a lazy expression may include other lazy expressions.

We implement lazy evaluation without any mutation or other destructive operations -- essentially in a call-by-value lambda-calculus with shift and reset.

Version
The current version is 1.2, Dec 28, 2008
References
lazy-eval.scm [6K]
The complete implementation and several illustrative examples, including the recursively-defined Fibonacci stream. The code was originally written on Feb 21, 2005. The current version adds a convenient automatic identification of lazy expressions and more examples.