sample code with fp-ts reader monads

reader-monad.ts

// ts 4.2.0
// fp-ts 2.9.5

import * as RE from 'fp-ts/lib/Reader';
import { pipe } from 'fp-ts/lib/pipeable';
import * as assert from 'assert';

interface Dependencies {
  logger: { log: (message: string) => void };
  env: 'development' | 'production';
}

function inner(n: number): RE.Reader<Dependencies, string> {
  return function({ env, logger }) {
    logger.log(`[${env}] calling inner`);
    return n.toString();
  };
}

function isEven(n: number): boolean {
  return n % 2 === 0;
}

function unrelated(s: string): string {
  return `${s}kg`;
}

function related(s: string): RE.Reader<Dependencies, string> {
  return function({ env }) {
    return `${s} in ${env}`;
  };
}

function sizeOf(s: string): number {
  return s.length;
}

const logger = {
  log: (message: string) => {
    console.log(message);
  },
};
const deps: Dependencies = { logger, env: 'development' };

let outer = pipe(
  inner(3),
  RE.map(unrelated),
  RE.chain(related),
  RE.map(sizeOf),
);

let result = outer(deps);
assert(result === 18);

// equivalent to the outer pipe: sizeOf(....)
const outer2 = pipe(
  RE.of(sizeOf),
  RE.ap(pipe(inner(3), RE.map(unrelated), RE.chain(related))),
);

result = outer2(deps);
assert(result === 18);