Assemble

A small but powerful functional programming utility for type-safe pipe-like operations.

Assemble composes arrays of functions that pick from and assign to a given type (assembling it). The composed "assembly" is like a pipe that steps through each function, merging input with any returned props and passing it to the next, returning the assembled result. Common use cases would be applying a sequence of functions to app state, DB or API results.

The focus of Assemble is to encourage an approach to function composition that is versatile yet simple to reason about and is type-safe with minimal definition overhead.

Hello World

This sample shows two "assemblers" operating on a props type. These functions benefit from type hinting for the props they can access and if defined, the props they need to return.

import { assemble, Assembler, VoidAssembler } from '@os-gurus/assemble'

interface Props {
  name: string
  message?: string
}

const prepareMessage: Assembler<Props, 'message'> = ({ name }) => {
  return { message: `Hello ${name}` }
}
// ☝️ Must return { message }

const logMessage: VoidAssembler<Props> = ({ message }) => {
  console.log(message)
}
// ☝️ Must return void

const sayHello = assemble(prepareMessage, logMessage)

sayHello({ name: 'World' })
// 🖨️ "Hello World"

Assembler Functions

The Assembler, PartialAssembler and VoidAssembler type utilities define functions that can be given to assemble and declare the props type they will operate on.

• Assembler functions return a subset of props, as defined by keys given to the utility.
• PartialAssembler functions optionally return a subset of props as defined.
• VoidAssembler functions return void, but can use the props, e.g. for logging or sending.

Note, you don't have to use these utilities. Any function signature can be given to assemble as long as it accepts a single props object argument and returns either a subset of props or nothing.

Examples of generated function definitions:

type Props = { message?: string, name?: string }

Assembler<Props, 'message'>
// ➥ (props: Props) => { message: string }

Assembler<Props, 'name' | 'message'>
// ➥ (props: Props) => { message: string, name: string }

PartialAssembler<Props, 'message'>
// ➥ (props: Props) => { message?: string } | undefined

VoidAssembler<Props>
// ➥ (props: Props) => void

Async Assemblers

AsyncAssembler works exactly as Assembler for asynchronous functions and assemble can compose a mixture of async and sync assemblers. As do AsyncPartialAssembler and AsyncVoidAssembler.

assembleSync can be used to enforce synchronous functions and a non-promise return.

import fetch from 'node-fetch'
import { assemble, Assembler, AsyncAssembler, VoidAssembler } from '@os-gurus/assemble'

interface Props {
  name?: string
  message?: string
}

const fetchName: AsyncAssembler<Props, 'name'> = async () => {
  const { results } = await fetch('https://randomuser.me/api').then(res => res.json())
  const { title, first } = results[0].name
  return { name: `${title} ${first}` }
}

const prepareMessage: Assembler<Props, 'message'> = ({ name }) => {
  return { message: `Hello ${name}` }
}

const logMessage: VoidAssembler<Props> = ({ message }) => {
  console.log(message)
}

const sayHello = assemble(fetchName, prepareMessage, logMessage)

sayHello({})
// 🖨️  "Hello {RANDOM_NAME}"

Mixing Function Types

Assemble will accept a range of function types and create an input type that merges all function prop types, while also returning the intersection of all input and assigned props. It can even take anonymous functions that are typed inline.

import { assembleSync, Assembler } from '@os-gurus/assemble'

type NumberProps = { one?: boolean, two?: boolean }
type LetterProps = { a?: boolean, b?: boolean }

const assignOne: Assembler<NumberProps, 'one'> = () => ({ one: true })
const assignA: Assembler<LetterProps, 'a'> = () => ({ a: true })

const mixedAssembly = assembleSync(
  assignOne,
  assignA,
  () => ({ foo: true }),
)
// ➥ Param { a?: boolean, b?: boolean, one?: boolean, two?: boolean }
// ☝️ All function prop types are merged into one

mixedAssembly({})
// ➥ Returns { a: boolean, b?: boolean, foo: boolean, one: boolean, two?: boolean }
// ☝️ Given assemblers provide `a` and `one` so they're no longer optional
// 💁‍♀️ Note `foo` wasn't in prop types but is inferred from the inline function

mixedAssembly({ b: true })
// ➥ Returns { a: boolean, b: true, foo: boolean, one: boolean, two?: boolean }
// ☝️ `b` is given so its prop type is narrowed to its literal value (true)

Known Issues

Merged type constraints override

Using MergeUnion on function's input and prop types to cast the return type of assemble creates a type error. Because 'Returned' could be instantiated with a different subtype of constraint '{}'. It is overridden by first casting to unknown. It can also be avoided by not merging props into one type and instead returning a union of all props, but that becomes unreadable for assemblies that have more than a few functions.

It would be nice to have a solution that avoids override casting, because there's a potential for it to return a different type at run time, which could cause false positive type checks.

Input shouldn't allow extra props

The Input type on assemble extends the Props type, but it should be more of an alias than an extension. The reason it's defined as a new type instead of just using Props is to tighten the return type by assigning whatever props are given as known literals instead of conditionals. The problem is that by extending, the Input type allows more props to be given than exist in Props.

The type hinting works well to suggest input props, but there should be a type error when giving props that aren't defined in Props. That would ease maintenance of compositions by making it obvious when props are given that are no longer required by its functions.

Does not infer mutable props

The assemble return type merges the union of assembler props, assemble input and assembler returns in that order. However, the input type is narrowed because it is given explicitly, which overrides its wider type in the props.

e.g. An Assembler returns { a: boolean } but assemble is given { a: true }, the return type will be { a: true } — In this case the assembler could override input and set { a: false } and the return type would be wrong.

It would be nice if it was possible for the assigned AssembledProps type to override the input even though it's wider, it would be safer.