ESLint Plugin fp-ts

0.5.0 verified Sat Apr 25 auth: no javascript

An ESLint plugin providing fp-ts-specific lint rules, including auto-fixable rules to detect deprecated imports (no-lib-imports, no-pipeable, no-module-imports) and to reduce redundant functional patterns (no-redundant-flow, prefer-traverse, prefer-chain, prefer-bimap), plus a rule to catch discarded pure expressions (no-discarded-pure-expression). Current stable version v0.5.0 requires ESLint ^9.0 and ships TypeScript definitions. The plugin has migrated to the new flat config format and now only supports ESLint v9, dropping support for v8. It relies on typescript-eslint for rules that need type checking.

Common errors

error Error [ERR_REQUIRE_ESM]: require() of ES Module ↓

cause Using require() to load the plugin when the package is ESM-only.

fix

Use import syntax: import fptsPlugin from 'eslint-plugin-fp-ts';

error Configuration for rule "fp-ts/no-discarded-pure-expression" is invalid: has no properties. ↓

cause Missing parser options for type-checking rules.

fix

Add languageOptions.parserOptions.projectService: true to your flat config.

error ESLint couldn't find the plugin "eslint-plugin-fp-ts". ↓

cause Plugin not installed or incorrectly referenced in config.

fix

Ensure eslint-plugin-fp-ts is installed and imported as shown in the quickstart.

Warnings

breaking Plugin v0.5.0 drops support for ESLint v8; only ESLint v9 flat config is supported. ↓

fix Upgrade ESLint to ^9.0 and migrate to flat config (eslint.config.mjs).

breaking Plugin v0.4.0 migrated to ESLint v9 and dropped CommonJS support; require() will fail. ↓

fix Use ESM imports and flat config or downgrade to v0.3.x if using eslintrc and CJS.

breaking Plugin v0.3.0 upgraded to ESLint v8, dropping ESLint v7 support. ↓

fix Upgrade ESLint to ^8.0 or use v0.2.x for ESLint v7.

gotcha Rules that require type-checking (e.g., no-discarded-pure-expression) will not work without configuring parserOptions.projectService or project. ↓

fix Set parserOptions.projectService: true (or project) and tsconfigRootDir in languageOptions.

deprecated Importing from 'fp-ts/lib/' is deprecated; use 'fp-ts/<module>' instead. ↓

fix Use rule 'fp-ts/no-lib-imports' to enforce the new import style.

Install

npm install eslint-plugin-fp-ts

yarn add eslint-plugin-fp-ts

pnpm add eslint-plugin-fp-ts

Imports

default
wrong
```
const fptsPlugin = require('eslint-plugin-fp-ts')
```
correct
```
import fptsPlugin from 'eslint-plugin-fp-ts'
```
The plugin is ESM-only since v0.5.0; require() will fail. Must use import for flat config.
no-lib-imports
wrong
```
import { 'no-lib-imports' } from 'eslint-plugin-fp-ts'
```
correct
```
import { noLibImports } from 'eslint-plugin-fp-ts/rules/no-lib-imports'
```
Individual rules can be imported from 'eslint-plugin-fp-ts/rules/<rule-name>', but the plugin object is the simplest approach.
defineConfig
wrong
```
import { defineConfig } from 'eslint'
```
correct
```
import { defineConfig } from 'eslint/config'
```
defineConfig is exported from 'eslint/config' in ESLint v9, not from the main 'eslint' package.
types
wrong
```
import { Plugin } from 'eslint'
```
correct
```
import type { Plugin } from 'eslint'
```
Type imports should use import type to avoid runtime side effects.

Quickstart

Flat config example enabling 'no-lib-imports', 'no-redundant-flow', and 'no-discarded-pure-expression' rules with parser options for type checking.

import { defineConfig } from 'eslint/config';
import fptsPlugin from 'eslint-plugin-fp-ts';

export default defineConfig({
  plugins: {
    'fp-ts': fptsPlugin,
  },
  languageOptions: {
    parserOptions: {
      projectService: true,
      tsconfigRootDir: import.meta.dirname,
    },
  },
  rules: {
    'fp-ts/no-lib-imports': 'error',
    'fp-ts/no-redundant-flow': 'error',
    'fp-ts/no-discarded-pure-expression': 'warn',
  },
});