prefer-chain.md

Replace map + flatten with chain (fp-ts/prefer-chain)

Suggest replacing the combination of map (or mapWithIndex) followed by flatten with chain (or chainWithIndex).

💡 Fixable: This rule provides in-editor suggested fixes.

Rule Details

Examples of incorrect code for this rule:

import { pipe } from "fp-ts/function";
import { map, flatten } from "fp-ts/Array";

pipe(
  [1, 2, 3],
  map((n) => [n, n + 1]),
  flatten
);

import { pipe } from "fp-ts/function";
import { mapWithIndex, flatten } from "fp-ts/Array";

pipe(
  [1, 2, 3],
  mapWithIndex((i, n) => [i, n]),
  flatten
);

Examples of correct code for this rule:

import { pipe } from "fp-ts/function";
import { chain } from "fp-ts/Array";

pipe(
  [1, 2, 3],
  chain((n) => [n, n + 1])
);

import { pipe } from "fp-ts/function";
import { chainWithIndex } from "fp-ts/Array";

pipe(
  [1, 2, 3],
  chainWithIndex((i, n) => [i, n])
);