Some fun with fp-ts2's Reader monad

package.json

{
  "name": "reader-intro",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "tsc",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "fp-ts": "^2.7.0",
    "ts-node": "^9.0.0",
    "typescript": "^3.9.7"
  }
}

v0-noDeps.ts

type ProductId = string

type ProductJSON = {
  productId: string,
  productName: string
}


export const getPdpData = (id: ProductId): ProductJSON => {
  return cacheOrClient(id)
}

export const cacheOrClient = (id: ProductId): ProductJSON => {
  if (false) {
    // get from cache
  } else {
    return getProductFromMicroservice(id)
  }
}

export const getProductFromMicroservice = (id: ProductId): ProductJSON => {
  console.log(`Making request to https://microservice/products/${id}`)
  return {
    productId: id,
    productName: "Super Cool Godzilla Socks"
  }
}

const result = getPdpData("E417963-000")
console.dir(result)

v1-withDeps.ts

type ProductId = string

type ProductJSON = {
  productId: string,
  productName: string
}

type Dependencies = {
  baseUrl: string
}

const getPdpData = (id: ProductId) => (deps: Dependencies): ProductJSON => {
  return cacheOrClient(id)(deps)
}

const cacheOrClient = (id: ProductId) => (deps: Dependencies): ProductJSON => {
  if (false) {
    // get from cache
  } else {
    return getProductFromMicroservice(id)(deps)
  }
}

const getProductFromMicroservice = (id: ProductId) => (deps: Dependencies): ProductJSON => {
  console.log(`Making request to ${deps.baseUrl}/products/${id}`)
  return {
    productId: id,
    productName: "Cool Product"
  }
}

const myDeps = {
  baseUrl: "https://microservice"
}

const result = getPdpData("E417963-000")(myDeps)
console.dir(result)

v2-withReader.ts

import * as Reader from "fp-ts/lib/Reader"

type ProductId = string

type ProductJSON = {
  productId: string,
  productName: string
}

type Dependencies = {
  baseUrl: string
}

// Controller?
export const getPdpData = (id: ProductId) => {
  return cacheOrClient(id)
}

export const cacheOrClient = (id: ProductId) => {
  if(false) {
    // Get from cache
  } else {
    return getProductFromMicroservice(id)
  }
}

// Fake http client
// getProductFromMicroservice :: ProductId -> Dependencies -> ProductJSON
// Reader<Dependencies, ProductJSON> :: Dependencies -> ProductJSON
// getProductFromMicroservice :: ProductId -> Reader<Dependencies, ProductJSON>
export const getProductFromMicroservice = (id: ProductId) => {
  const f = (deps: Dependencies) => {
    console.log(`Making request to ${deps.baseUrl}/products/${id}`)
    return {
      productId: id,
      productName: "Super Cool Godzilla Socks"
    }
  }
  
  const getDeps = Reader.ask<Dependencies>()
  return Reader.map(f)(getDeps)

}


const myDeps = {
  baseUrl: "https://microservice"
}
const result = getPdpData("E417963-000")(myDeps)
console.dir(result)