# hazelgrove/hazel

Hazel, a live functional programming environment with typed holes

Repository: https://github.com/hazelgrove/hazel
Canonical: https://ross.abutalabs.com/products/hazelgrove-hazel
Homepage: http://hazel.org/
Language: Reason
License: MIT
License Family: permissive
Topics: editor, functional-programming, ocaml, elm, reasonml, holes
Last push: 2026-09-02T22:03:42+00:00

## Health v2 (maintenance only)
Score: 68/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 100, release rhythm 8, longevity 100
- inputs: {"age_days": 3506, "days_push": 0, "days_rel": 422, "gap_med": null, "n_releases_24m": 1}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1142, forks 67 (observed 2026-09-03T02:15:12.648745+00:00)

## What it is
Hazel is a web-based live functional programming environment for an Elm/ML-like language built around typed holes, allowing typechecking, editing, and even running incomplete programs. It is a research project implemented in Reason/OCaml and compiled to the browser, with no meaningless editor states.

## Use cases
- experiment with typed-hole-driven development in a functional language
- try a live programming environment that runs incomplete programs
- explore research on semantic foundations for program editors
- teach type theory and functional programming interactively
- edit functional programs without ever hitting invalid editor states

## When to choose
- you want to experiment with or contribute to research on typed holes and live programming
- you need an environment that gives feedback on incomplete or erroneous programs
- you are teaching or learning functional programming with immediate type feedback

## When to avoid
- you need a production-ready IDE for mainstream languages
- you want a stable tool with long-term ecosystem support
- you need offline desktop tooling or integration with existing compilers

## Facets
- artifact type: application
- maturity: active
- function: editor, interpreter, type-system, programming-language
- domain: programming-languages, developer-tools, text-editors, education
- platform: browser, cross-platform
- tags: typed-holes, functional-programming, live-programming, reasonml, ocaml, research-project, web-based-ide, web-server

## Member repositories
- hazelgrove/hazel (main) score 68

## Provenance
- Observed fields: from GitHub, fetched 2026-09-03T02:15:12.648745+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T06:35:30.694099+00:00, confidence not recorded.
  - readme: https://github.com/hazelgrove/hazel (fetched 2026-09-03T02:15:12.648745+00:00, sha d43c38fe5250)
  - homepage: http://hazel.org/ (fetched 2026-08-29T12:40:40.764077+00:00, sha c166b9f33347)
- Data as of 2026-08-30T08:39:29.467469+00:00.