Hazel is a live functional programming environment organized around typed holes.

Quick Summary

Motivation

When programming, we spend a substantial amount of our time working with program text that is not yet a formally complete program, e.g. because there are blank spots, type errors or merge conflicts at various locations.

Conventional programming language definitions assign no formal meaning to structures like these, so we are left without live feedback about the behavior of even complete portions of the program. Moreover, program editors and other tools have no choice but to resort to complex and ad hoc heuristics to provide various useful language services (like code completion, type inspection, and code navigation) without gaps in service.

We are developing a more principled approach to working with incomplete programs, rooted in (contextual modal and gradual) type theory. We model incomplete programs as programs with holes, which (1) stand for parts of the program that are missing; and (2) serve as membranes around parts of the program that are erroneous or, in the collaborative setting, conflicted.

The following papers describes our research vision in more detail.

Hazel

We are first implementing these ideas into Hazel, a web-based programming environment for an Elm/ML-like functional programming language designed around typed-hole-driven development.

Uniquely, every incomplete program that you can construct using Hazel's language of edit actions is both statically and dynamically well-defined, i.e. it has a (possibly incomplete) type, and you can run it to produce a (possibly incomplete) result. Consequently, Hazel serves as an elegant platform for research on the future of programming (and programming education).

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