F# : Onion architecture in a nutshell

onion-1.fs

// 1. pure, don't think about IO at all
module Domain =
    let add x y = x + y

// 2. think about IO but not its implementation
module App =
    let add (getX: unit -> Async<int32>) y =
        async {
            let! x = getX ()
            return Domain.add x y
        }

// 3. IO implementation
module Infra =
    let getX () = async { return 7 }

// 4. inject dependencies
module Startup =
    let add = App.add Infra.getX

// demo
Startup.add 3
|> Async.RunSynchronously
|> printfn "%A" // 10

onion-2.fs

// 1. pure, don't think about IO at all
module Domain =
    let add x y = x + y

// 2. think about IO but not its implementation
module App =
    let add (getX: unit -> Async<int32>) y =
        async {
            let! x = getX ()
            return Domain.add x y
        }

// 3. IO implementation
module Infra =
    open System.Data.SqlClient
    let newConnection () = new SqlConnection ()
    let getX conn = async { return 7 }

// 4. inject dependencies
module Startup =
    let add y =
        async {
            use conn = Infra.newConnection ()
            return! App.add (fun () -> Infra.getX conn) y
        }

// demo
Startup.add 3
|> Async.RunSynchronously
|> printfn "%A" // 10

onion_1.ts

// 1_domain.ts
// 1. pure, don't think about IO at all

export function add(x: number, y: number) {
  return x+y;
}

// 2_app.ts
// 2. think about IO but not its implementation
import * as Domain from './1_domain'

export async function add( getX: () => Promise<number>, y: number) {
  const x = await getX()
  return Domain.add(x, y);
}

// 3_infra.ts
// 3. IO implementation
import fetch from 'node-fetch';

export async function getX(apiUrl:string): Promise<number> {
  const response = await fetch(apiUrl, { method: 'GET'});
  const data = await response.json();
  if (response.ok) {
    return (data as number);
  } else {
    return Promise.reject(`Got an error response ${JSON.stringify(data)}`);
  }
}

export function getConfigurationUrl() : string {
  throw new Error('Function not implemented.');
}

// 4_index.ts
import * as App from './2_app'
import * as Infra from './3_infra'

// 4. inject dependencies
export function add (y:number){
  const apiUrl = Infra.getConfigurationUrl();
  return App.add ( ()=> Infra.getX(apiUrl), y);
}