# com-lihaoyi/fastparse

Writing Fast Parsers Fast in Scala

Repository: https://github.com/com-lihaoyi/fastparse
Canonical: https://ross.abutalabs.com/products/fastparse
Homepage: https://com-lihaoyi.github.io/fastparse
Language: Scala
License: MIT
License Family: permissive
Last push: 2026-08-20T13:44:53+00:00

## Health v2 (maintenance only)
Score: 67/100 (v2, computed 2026-09-02T17:46:02.011165+00:00)
- activity 98, release rhythm 8, longevity 100
- inputs: {"age_days": 4295, "days_push": 13, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1135, forks 164 (observed 2026-08-28T04:03:43.364442+00:00)

## What it is
FastParse is a Scala parser-combinator library for parsing strings and bytes into structured data, with hand-written-parser-level speed and excellent error reporting. It works on both Scala JVM and Scala.js and includes example parsers for Scala, Python, and CSS.

## Use cases
- parse custom text file formats in Scala
- build a parser for a programming language or DSL
- parse JSON-like structured data with good error messages
- write fast parser combinators instead of regex hacks
- parse source code in the browser via Scala.js
- stream-parse large datasets without loading everything into memory

## When to choose
- you need a fast, maintainable parser in Scala with great diagnostics
- you want parser combinators far faster than scala-parser-combinators
- you need parsing on both JVM and JavaScript via Scala.js

## When to avoid
- you need a parser generator producing standalone grammars like ANTLR
- you are not working in Scala or a JVM/Scala.js ecosystem
- you need extreme-performance binary parsing beyond combinator overhead

## Facets
- artifact type: library
- maturity: stable
- function: parser
- domain: parsers, programming-languages, developer-tools
- platform: jvm, browser, cross-platform
- tags: parser-combinators, scala-js, scala, error-reporting, text-parsing

## Member repositories
- com-lihaoyi/fastparse (main) score 67

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:03:43.364442+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 2026-08-30T06:36:46.505229+00:00, confidence not recorded.
  - readme: https://github.com/com-lihaoyi/fastparse (fetched 2026-08-28T04:03:43.364442+00:00, sha 6dfdf3a7fcf5)
  - homepage: https://com-lihaoyi.github.io/fastparse (fetched 2026-08-29T12:41:40.410686+00:00, sha 71605eb51433)
- Data as of 2026-08-30T08:39:29.467469+00:00.