Effect

Table of Contents

1. Overview
2. Example
3. Installation
4. Usage
   • Basic Construction
   • Chaining Effects
   • Handling Promises
   • Pure Effects

Overview

A small library for handling side effects! Particularly promises in Gleam.

Inspired by Lustre’s Effects and effect-ts.

This library allows you to treat async operations and potential failures as first-class effectful computations, which can be composed before finally being executed.

The motivation was to create an API for dealing with promises from gleam_promises.

Example

import gleam/fetch
import gleam/io
import gleam/javascript/promise
import gleam/result
import gleam/string

import effect.{type Effect}
import effect/promise as effect_promise

pub type Error {
  UriParse
  Fetch(fetch.FetchError)
}

pub fn main() {
  let google: Effect(String, Error) = {
    use uri <- effect.from_result_map_error(
      uri.parse("https://www.google.com"),
      fn (_) { UriParse }, // replacing the error
    )
    use req <- effect.from_result_map_error(
      request.from_uri(uri),
      fn (_) { UriParse }
    )
    use resp <- effect_promise.from_promise_result(fetch.send(req), Fetch)
    use text <- effect_promise.from_promise_result(
      fetch.read_text_body(resp),
      Fetch,
    )
    text.body |> effect.continue
  }

  use res: Result(String, Error) <- effect.perform(google)
  case res {
    Ok(body) -> body |> io.println
    Error(e) -> e |> string.inspect |> io.println_error
  }
}

Further documentation can be found at https://hexdocs.pm/effect.

Installation

gleam add effect@2

Usage

Below are demonstrations of common usages demonstrating on how to create and compose effects, handle failures, work with promises, and actually perform the effect.

Basic Construction

import gleam/int
import gleam/io

import effect.{type Effect}

pub fn main() {
  // Succeed / Fail shortcuts
  // Effect(Int, early)
  let eff_ok = effect.continue(42)
  // Effect(msg, String)
  let eff_err = effect.throw("Oops!")

  // Combine
  let eff: Effect(Int, String) = {
    case int.random(2) |> int.is_even {
      True -> eff_ok
      False -> eff_err
    }
  }

  // Perform
  use res: Result(Int, String) <- effect.perform(eff)

  case res {
    Ok(num) -> io.println("Got: " <> int.to_string(num))
    Error(err) -> io.println("Error: " <> err)
  }
}

Chaining Effects

Use then and map to sequence operations, only continuing on success, or short-circuiting on error:

import gleam/float
import gleam/int
import gleam/io

import effect.{type Effect}

type TooSmallError {
  TooSmallError
}

pub fn main() {
  let computation: Effect(Int, TooSmallError) = {
    let effect: Effect(Int, TooSmallError) = effect.continue(10)
    use num: Int <- effect.then(effect)
    case num < 5 {
      True -> effect.throw(TooSmallError)
      False -> effect.continue(num * 2)
    }
  }

  let computation: Effect(Float, TooSmallError) = {
    use num: Int <- effect.map(computation)
    let f: Float = int.to_float(num)
    // fun fact, I added this func to std
    let log: Float = float.exponential(f)
  }

  use res: Result(Float, TooSmallError) <- effect.perform(computation)
  case res {
    Ok(n) -> io.println("Final result: " <> float.to_string(n))
    Error(TooSmallError) -> io.println("Error: num is too small!")
  }
}

Handling Promises

You can convert a promise.Promise(Result(a, e)) into an Effect(a, e) using from_promise_result.

import gleam/fetch.{type FetchBody, type FetchError}
import gleam/javascript/promise.{type Promise}
import gleam/string

import effect.{type Effect}
import effect/promise as effect_promise

type Error {
  Fetch(fetch.FetchError)
}

fn fetch_data() -> Promise(Result(FetchBody, FetchError)) {
  let assert Ok(uri) = uri.parse("https://www.google.com")
  let assert Ok(req) = request.from_uri(uri)
  fetch.send(req)
}

fn main() {
  let eff: Effect(String, Error) = {
    // fetch from google
    let prom = fetch_data_readme()
    // get an effect from a promise(result)
    use fetch_body: Response(FetchBody) <- effect_promise.from_promise_result(
      prom,
      Fetch, // map the error
    )

    // read the response from FetchBody to String
    let prom: Promise(Result(Response(String), FetchError)) =
      fetch.read_text_body(fetch_body)

    use text: Response(String) <- effect_promise.from_promise_result(
      prom,
      // map the error
      Fetch,
    )

    // return just the body
    text.body |> effect.continue
  }

  use res: Result(String, Error) <- effect.perform(eff)
  case res {
    // Server Data
    Ok(body) -> io.println(body)
    // Network Error
    Error(Fetch(msg)) -> io.println("Failed: " <> string.inspect(msg))
    _ -> io.println("Failed: other error")
  }
}

Pure Effects

Sometimes you don’t need the early return path and only need to operate on the happy path. The perform function returns a Result type but there’s a pure alternative:

import gleam/int
import gleam/io

import effect.{type Effect}

fn main() {
  let eff: Effect(Int, effect.Nothing) = {
    use a <- effect.from(5)
    use b <- effect.from(2)
    a * b |> effect.continue
  }

  use num: Int <- effect.pure(eff)
  num |> int.to_string |> io.println
}

For more, be sure to checkout the test cases.