[Functional Programming] Reader with Async ADT

Posted 2021-12-14 answer1215

ReaderT is a Monad Transformer that wraps a given Monad with a Reader. This allows the interface of a Reader that enables the composition of computations that depend on a shared environment (e -> a), but provides a way to abstract a means the Reader portion, when combining ReaderTs of the same type. All ReaderTs must provide the constructor of the target Monad that is being wrapped.

 

The task we want to do is read from a "data.json" file:

  "name": "App",
  "config": 
    "prod": "config.prod.json",
    "dev": "config.dev.json",
    "test": "config.test.json"
  

 

According to the ‘env‘ variable we pass in, it will pick different config file:

config.test.json:

  "port": 5200

 

In the end, it will output a json file combine the result.

 

const  readJSON, writeJSON, fork  = require("./helper");
const 
  Async,
  ReaderT,
  omit,
  pipeK,
  assign
 = require("crocks");

const ReaderAsync = ReaderT(Async);
const env = 
  input: "data.json",
  output: "output.json"
;

const input = env =>
  ReaderAsync(( input ) => readJSON(input).map(assign( env )));
const config = data =>
  ReaderAsync(() =>
    readJSON(data.config[data.env])
      .map(assign(data))
      .map(omit(["config"]))
  );
const output = inputData =>
  ReaderAsync(( output ) => writeJSON(output, inputData));
const flow = pipeK(
  ReaderAsync.of,
  input,
  config,
  output
);

fork(flow("test").runWith(env));

　　

output.json file:

  "port": 5200,
  "name": "App",
  "env": "test"