A set of types and functions to make compositions easy and safe.

• 🛟 Type-Safe Compositions: Ensure robust type-safety during function composition, preventing incompatible functions from being combined and reducing runtime errors.
• 🔄 Promise and Error Handling: Focus on the happy-path of your functions eliminating the need for verbose try/catch syntax.
• 🏝️ Isolated Business Logic: Split your code into composable functions, making your code easier to test and maintain.
• 🔒 End-to-End Type Safety: Achieve end-to-end type safety from the backend to the UI with serializable results, ensuring data integrity across your entire application.
• ⚡ Parallel and Sequential Compositions: Compose functions both in parallel - with all and collect - and sequentially - with pipe, branch, and sequence -, to manage complex data flows optimizing your code for performance and clarity.
• 🕵️‍♂️ Runtime Validation: Use withSchema or applySchema with your favorite parser for optional runtime validation of inputs and environments, enforcing data integrity only when needed.
• 🚑 Resilient Error Handling: Leverage enhanced combinators like mapErrors and catchFailure to transform and handle errors more effectively.
• 📊 Traceable Compositions: Use the trace function to log and monitor your composable functions’ inputs and results, simplifying debugging and monitoring.

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Table of contents

• Quickstart
• Composing type-safe functions
• Creating primitive composables
• Sequential composition
  • Using non-composables (mapping)
• Parallel composition
• Handling errors
  • Throwing
  • Catching
  • Mapping the errors
• Unwrapping the result
• Guides
  • Migrating from domain-functions
  • Handling external input
  • Defining constants for multiple functions (environments)
  • Using custom parsers
• Using Deno
• Acknowledgements

Quickstart

npm i composable-functions

import { composable, pipe } from 'composable-functions'

const faultyAdd = composable((a: number, b: number) => {
  if (a === 1) throw new Error('a is 1')
  return a + b
})
const show = composable(String)
const addAndShow = pipe(faultyAdd, show)

const result = await addAndShow(2, 2)
/*
result = {
  success: true,
  data: "4",
  errors: []
}
*/
const failedResult = await addAndShow(1, 2)
/*
failedResult = {
  success: false,
  errors: [<Error object>]
}
*/

Composing type-safe functions

Let's say we want to compose two functions: add: (a: number, b:number) => number and toString: (a: number) => string. We also want the composition to preserve the types, so we can continue living in the happy world of type-safe coding. The result would be a function that adds and converts the result to string, something like addAndReturnString: (a: number, b: number) => string.

Performing this operation manually is straightforward

function addAndReturnString(a: number, b: number): string {
  return toString(add(a, b))
}

It would be neat if typescript could do the typing for us and provided a more generic mechanism to compose these functions. Something like what you find in libraries such as lodash

Using composables the code could be written as:

const addAndReturnString = pipe(add, toString)

We can also extend the same reasoning to functions that return promises in a transparent way. Imagine we have add: (a: number, b:number) => Promise<number> and toString: (a: number) => Promise<string>, the composition above would work in the same fashion, returning a function addAndReturnString(a: number, b: number): Promise<string> that will wait for each promise in the chain before applying the next function.

This library also defines several operations besides the pipe to compose functions in arbitrary ways, giving a powerful tool for the developer to reason about the data flow without worrying about mistakenly connecting the wrong parameters or forgetting to unwrap some promise or handle some error along the way.

Creating primitive composables

A Composable is a function that returns a Promise<Result<T>> where T is any type you want to return. Values of the type Result will represent either a failure (which carries a list of errors) or a success, where the computation has returned a value within the type T.

So we can define the add and the toString functions as a Composable:

import { composable } from 'composable-functions'

const add = composable((a: number, b: number) => a + b)
//    ^? Composable<(a: number, b: number) => number>

const toString = composable((a: unknown) => `${a}`)
//    ^? Composable<(a: unknown) => string>

Sequential composition

Now we can compose them using pipe to create addAndReturnString:

import { pipe } from 'composable-functions'

const addAndReturnString = pipe(add, toString)
//    ^? Composable<(a: number, b: number) => string>

Note that trying to compose pipe flipping the arguments will not type-check:

import { pipe } from 'composable-functions'

const addAndReturnString = pipe(toString, add)
//    ^? Internal.FailToCompose<string, number>

Since pipe will compose from left to right, the only string output from toString will not fit into the first argument of add which is a number. The error message comes in the form of an inferred FailToCompose type, this failure type is not callable therefore it will break any attempts to call addAndReturnString.

Using non-composables (mapping)

Sometimes we want to use a simple function in this sort of sequential composition. Imagine that toString is not a composable, and you just want to apply a plain old function to the result of add when it succeeds. The function map can be used for this, since we are mapping over the result of a Composable:

import { map } from 'composable-functions'

const addAndReturnString = map(add, result => `${result}`)

Parallel composition

There are also compositions where all functions are excuted in parallel, like Promise.all will execute several promises and wait for all of them. The all function is one way of composing in this fashion. Assuming we want to apply our add and multiply the two numbers returning a success only once both operations succeed:

import { composable, all } from 'composable-functions'

const add = composable((a: number, b: number) => a + b)
const mul = composable((a: number, b: number) => a * b)
const addAndMul = all(add, mul)
//    ^? Composable<(a: number, b: number) => [number, number]>

The result of the composition comes in a tuple in the same order as the functions were passed to all. Note that the input functions will also have to type-check and all the functions have to work from the same input.

Handling errors

Since a Composable always return a type Result<T> that might be either a failure or a success, there are never exceptions to catch. Any exception inside a Composable will return as an object with the shape: { success: false, errors: Error[] }.

Two neat consequences is that we can handle errors using functions (no need for try/catch blocks) and handle multiple errors at once.

Throwing

You can throw anything derived from Error. Check this documentation on how to define your custom errors. The library will also wrap anything that does not extends Error, just to keep compatibility with code-bases that throw strings or objects.

import { composable } from 'composable-functions'

class NotFoundError extends Error {
  constructor(message) {
    super(message);
    this.name = 'NotFoundError';
  }
}

const getUser = composable((userId: string, users: Array<string>) => {
//    ^? Composable<(userId: string, users: Array<string>) => string>
    const result = users.find(({id}) => id === userId)
    if(result == undefined) throw new NotFoundError(`userId ${userId} was not found`)
    return result
})

The library defines a few custom errors out of the box but these will be more important later on, whend dealing with external input and schemas. See the errors module for more details.

Catching

You can catch an error in a Composable, using catchFailure which is similar to map but will run whenever the first composable fails:

import { composable, catchFailure } from 'composable-functions'

// assuming we have the definitions from the previous example
const getOptionalUser = catchFailure(getUser, (errors, id) => {
  console.log(`Failed to fetch user with id ${id}`, errors)
  return null
})
//    ^? Composable<(id: string) => string | null>

Mapping the errors

Sometimes we just need to transform the errors into something that would make more sense for the caller. Imagine you have our getUser defined above, but we want a custom error type for when the ID is invalid. You can map over the failures using mapErrors and a function with the type (errors: Error[]) => Error[].

import { mapErrors } from 'composable-functions'

class InvalidUserId extends Error {}
const getUserWithCustomError = mapErrors(getUser, (errors) =>
  errors.map((e) => e.message.includes('Invalid ID') ? new InvalidUserId() : e)
)

Unwrapping the result

Keep in mind the Result type will only have a data property when the composable succeeds. If you want to unwrap the result, you must check for the success property first.

const result = await getUser('123')
if (!result.success) return notFound()

return result.data
//            ^? User

TypeScript won't let you access the data property without checking for success first, so you can be sure that you are always handling the error case.

const result = await getUser('123')
// @ts-expect-error: Property 'data' does not exist on type 'Result<User>'
return result.data

You can also use fromSuccess to unwrap the result of a composable that is expected to always succeed. Keep in mind that this function will throw an error if the composable fails so you're losing the safety layer of the Result type.

const fn = composable(async (id: string) => {
  const valueB = await fromSuccess(anotherComposable)({ userId: id })
  // do something else
  return { valueA, valueB }
})

We recomend only using fromSuccess when you are sure the composable must succeed, like when you are testing the happy path of a composable.

You can also use it within other composables whenever the composition utilities fall short, the error will be propagated as ErrorList and available in the caller Result.

const getUser = composable((id: string) => db().collection('users').findOne({ id }))

const getProfile = composable(async (id: string) => {
  const user = await fromSuccess(getUser)(id)
  // ... some logic
  return { user, otherData }
})

Guides

Migrating from domain-functions

Handling external input

Defining constants for multiple functions (environments)

Using custom parsers

Using Deno

If you are using Deno, just directly import the functions you need from deno.land/x:

import { composable } from "https://deno.land/x/composable_functions/mod.ts";

This documentation will use Node.JS imports by convention, just replace composable-functions with https://deno.land/x/composable_functions/mod.ts when using Deno.

Acknowledgements

Composable Functions' logo by NUMI: