polysemy-research/polysemy

Health v2 · maintenance only

Adoption not part of the score

1072 stars · 77 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-30, confidence not recorded

Polysemy is a Haskell library for building higher-order, low-boilerplate effect systems (extensible effects), letting you write domain-specific languages that separate business logic from implementation details. It composes better than mtl with no O(n^2) instances problem and supports multiple copies of the same effect.

Use cases

• write effectful Haskell code without mtl boilerplate
• separate business logic from implementation details in Haskell
• define custom effects with minimal boilerplate
• use multiple instances of the same effect like two Reader environments
• model typed errors without ugly hacks
• write higher-order effects like bracket and local
• port code from transformers/mtl style to an effects system

When to choose

• you're writing Haskell and want composable effects without functional dependencies
• you need multiple copies of the same effect in one stack
• you want to define new effects in a few lines and interpreters as plain functions
• you're tired of mtl's O(n^2) instances problem

When to avoid

• you're not programming in Haskell
• you need maximum runtime performance, as polysemy's interpreter overhead can be significant
• your team already has a mature mtl or fused-effects codebase
• you want a library with a large ecosystem of prebuilt effects

Facets

library · maturity stable

programming-language developer-tools programming-languages developer-tools jvm haskell effects-system monads free-monads extensible-effects higher-order-effects dsl algorithms

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