# typelevel/cats

Lightweight, modular, and extensible library for functional programming.

Repository: https://github.com/typelevel/cats
Canonical: https://ross.abutalabs.com/products/cats
Homepage: https://typelevel.org/cats/
Language: Scala
License: NOASSERTION
License Family: other
Last push: 2026-08-21T12:13:32+00:00

## Health v2 (maintenance only)
Score: 67/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 98, release rhythm 8, longevity 100
- inputs: {"age_days": 4235, "days_push": 12, "days_rel": 590, "gap_med": null, "n_releases_24m": 1}
- flags: no_license
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 5456, forks 1245 (observed 2026-08-28T04:09:19.330069+00:00)

## What it is
Cats is a lightweight, modular, and extensible Scala library providing abstractions for functional programming, such as type classes (Functor, Monad, Applicative) and pure data types. It aims to be a stable, binary-compatible foundation for an ecosystem of pure, typeful Scala libraries, running on JVM, Scala.js, and Scala Native.

## Use cases
- use type classes like Monad and Functor in Scala
- write pure functional Scala code
- model errors with validated and Either
- build a pure FP library ecosystem on a stable core
- learn functional programming abstractions in Scala
- replace Scalaz with a modular FP foundation

## When to choose
- you are doing functional programming in Scala and want lawful, modular type classes
- you need a stable, binary-compatible FP core for Scala libraries
- you want cross-building support across JVM, Scala.js, and Scala Native

## When to avoid
- you want a batteries-included FP standard library with IO/Task included (look at Cats Effect or ZIO)
- your project is imperative/OO Scala with no interest in FP abstractions
- you need a runtime effect system rather than pure abstractions

## Facets
- artifact type: library
- maturity: stable
- function: type-system, developer-tools
- domain: programming-languages, developer-tools
- platform: jvm, cross-platform
- tags: functional-programming, typeclasses, category-theory, scala, scalajs, scala-native, pure-fp, algorithms

## Member repositories
- typelevel/cats (main) score 67

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:09:19.330069+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 2026-08-29T17:56:48.003852+00:00, confidence not recorded.
  - readme: https://github.com/typelevel/cats (fetched 2026-08-28T04:09:19.330069+00:00, sha 6c86cb87ecd5)
  - homepage: https://typelevel.org/cats/ (fetched 2026-08-29T08:51:51.336633+00:00, sha 1bdd2ce73551)
  - site_page: https://typelevel.org/cats/faq.html (fetched 2026-08-29T08:51:51.345690+00:00, sha a0fbc9776983)
- Data as of 2026-08-30T08:39:29.467469+00:00.