Live functional programming with typed holes

Abstract

Live programming environments aim to provide programmers (and sometimes audiences) with continuous feedback about a program's dynamic behavior as it is being edited. The problem is that programming languages typically assign dynamic meaning only to programs that are complete, i.e. syntactically well-formed and free of type errors. Consequently, live feedback presented to the programmer exhibits temporal or perceptive gaps. This paper confronts this "gap problem" from type-theoretic first principles by developing a dynamic semantics for incomplete functional programs, starting from the static semantics for incomplete functional programs developed in recent work on Hazelnut. We model incomplete functional programs as expressions with holes, with empty holes standing for missing expressions or types, and non-empty holes operating as membranes around static and dynamic type inconsistencies. Rather than aborting when evaluation encounters any of these holes as in some existing systems, evaluation proceeds around holes, tracking the closure around each hole instance as it flows through the remainder of the program. Editor services can use the information in these hole closures to help the programmer develop and confirm their mental model of the behavior of the complete portions of the program as they decide how to fill the remaining holes. Hole closures also enable a fill-and-resume operation that avoids the need to restart evaluation after edits that amount to hole filling. Formally, the semantics borrows machinery from both gradual type theory (which supplies the basis for handling unfilled type holes) and contextual modal type theory (which supplies a logical basis for hole closures), combining these and developing additional machinery necessary to continue evaluation past holes while maintaining type safety. We have mechanized the metatheory of the core calculus, called Hazelnut Live, using the Agda proof assistant.We have also implemented these ideas into the Hazel programming environment. The implementation inserts holes automatically, following the Hazelnut edit action calculus, to guarantee that every editor state has some (possibly incomplete) type. Taken together with this paper's type safety property, the result is a proof-of-concept live programming environment where rich dynamic feedback is truly available without gaps, i.e. for every reachable editor state.

Program sketching with live bidirectional evaluation

“…Then, the case expression in unJustÐwe write unJust to refer to its two branchesÐscrutinizes the hole closure. The form of the constructor application has not yet been determined, so evaluation łpausesž by returning the indeterminate [Omar et al 2019] result case ([−] ?? 5 ) unJust, which records the fact that, when the scrutinee resumes to a constructor head Nothing or Just, evaluation of the case will proceed down the appropriate branch.…”

“…Live evaluation is adapted from Omar et al [2019] to our setting and is not a technical contribution of our work. Live unevaluation is the key novel mechanism thatÐtogether with live evaluationÐ enables us to łcombine sketching with Myth-style synthesisž (hence the name Smyth).…”

“…• We present live unevaluation, a novel technique that checks example satisfaction of sketches. The combination of live evaluation to partially evaluate sketches [Omar et al 2019] and live unevaluationÐwhich we call live bidirectional evaluationÐforms a core guess-and-check strategy for programs with holes. Our formulation generalizes Myth, but the notion of live bidirectional evaluation can also be developed for other evaluator-based synthesizers.…”

Focused Live Programming with Loop Seeds

“…Live programming is an area of research whose history dates back to the seminal work of Hancock [7] that introduced many important concepts in live programming. Since then, a variety of live environments has been developed for various languages, including Python [6,10], Java [3], Javascript [17,1], Lisp [2] and ML-like languages [16]. As the feld developed, several essays were published on categorizing different kinds of liveness [19,21], and several studies explored the benefts of live programming [22,13].…”

Programming Metamorphic Algorithms: An Experiment in Type-Driven Algorithm Design

“…As a result, the programmer gains better 'situation awareness' of what a program means while constructing the program. There are other desirable forms of situation awareness, for example the ability to execute incomplete programs offered by Hazel [27], and in general it would be interesting to develop more mechanisms to improve the programmer's situation awareness.…”