@lucas-bur/effect-memfs

@lucas-bur/effect-memfs

Platform-agnostic in-memory file system for Effect v4. Everything is kept in RAM – no disk, no native bindings. Works in Node, browsers, edge runtimes, anywhere.

Ideal for testing, mocking, and scenarios where you need a filesystem but don't want to touch the real one. Exposes the standard FileSystem service so your Effect code doesn't know the difference.

Install

npm install @lucas-bur/effect-memfs effect

Usage

import { Effect, FileSystem } from "effect"
import * as MemoryFileSystem from "@lucas-bur/effect-memfs"

const program = Effect.gen(function* () {
  const fs = yield* FileSystem.FileSystem
  yield* fs.writeFileString("hello.txt", "world")
  return yield* fs.readFileString("hello.txt")
})

Effect.runPromise(program.pipe(Effect.provide(MemoryFileSystem.layer())))
// => "world"

Pre-populate files:

import { Effect, FileSystem } from "effect"
import { layer } from "@lucas-bur/effect-memfs"

const program = Effect.gen(function* () {
  const fs = yield* FileSystem.FileSystem
  return yield* fs.readFileString("/src/index.ts")
})

Effect.runPromise(
  program.pipe(
    Effect.provide(
      layer({
        src: {
          "index.ts": "export const x = 1",
          utils: { "math.ts": "export const add = (a, b) => a + b" },
        },
        test: {
          "foo.test.ts": "import { x } from '../src'",
        },
        assets: null, // empty directory
      }),
    ),
  ),
)

License

MIT