WIP reader

exercises/reader.test.ts

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+import * as Read from './reader'
+import * as R from 'fp-ts/Reader'
+import * as M from 'fp-ts/Monoid'
+
+describe('reader', () => {
+  it('one', () => {
+    expect(Read.one([R.of(1), R.of(2), R.of(3)])(true)).toEqual(6)
+    expect(Read.one([])(true)).toEqual(0)
+  })
+  it('two', () => {
+    expect(
+      Read.two([R.of(1), R.of(2), R.of(3)], M.monoidSum)(true)
+    ).toEqual(6)
+    expect(Read.two([], M.monoidSum)(true)).toEqual(0)
+
+    const reader2 = Read.two(
+      [R.of(1), R.of(2), R.of(3), (a: number) => a + 1],
+      M.monoidProduct
+    )
+    expect(reader2(10)).toEqual(66)
+    expect(reader2(-1)).toEqual(0)
+  })
+  it('three', () => {
+    const reader = Read.three(
+      a => b => `${a} + ${b}`,
+      (rA: number) => rA * 2,
+      (rB: number) => rB * 3
+    )
+    expect(reader(0)).toEqual('0 + 0')
+    expect(reader(2)).toEqual('4 + 6')
+  })
+  it('four', () => {
+    const translationsEnglish = {
+      greeting: 'Good morning',
+      happyBirthday: 'Happy birthday',
+      genericPleasantry: 'Have a nice day',
+    }
+    const reader = Read.four('Mr Horse', new Date('2020-01-01'))
+    expect(
+      reader({
+        translations: translationsEnglish,
+        date: new Date('2020-01-01'),
+      })
+    ).toEqual('Good morning, Mr Horse! Happy birthday!')
+    expect(
+      reader({
+        translations: translationsEnglish,
+        date: new Date('2021-02-01'),
+      })
+    ).toEqual('Good morning, Mr Horse! Have a nice day.')
+
+    const translationsItalian = {
+      greeting: 'Buongiorno',
+      happyBirthday: 'Buon compleanno',
+      genericPleasantry: 'Buona giornata',
+    }
+    expect(
+      reader({
+        translations: translationsItalian,
+        date: new Date('2020-01-01'),
+      })
+    ).toEqual('Buongiorno, Mr Horse! Buon compleanno!')
+    expect(
+      reader({
+        translations: translationsItalian,
+        date: new Date('2021-02-01'),
+      })
+    ).toEqual('Buongiorno, Mr Horse! Buona giornata.')
+  })
+})

exercises/reader.ts

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+import * as R from 'fp-ts/Reader'
+import * as M from 'fp-ts/Monoid'
+import { pipe } from 'fp-ts/function'
+import * as A from 'fp-ts/Apply'
+
+// given an array of Reader values, return one return value with all the values
+// inside summed
+export const one = <R>(
+  as: R.Reader<R, number>[]
+): R.Reader<R, number> => pipe(as, M.fold(R.getMonoid(M.monoidSum)))
+
+// given an array of Reader values, and a Monoid that knows how to combine the
+// values, return a single Reader value with all the contents combined
+export const two = <R, A>(
+  as: R.Reader<R, A>[],
+  monoid: M.Monoid<A>
+): R.Reader<R, A> => pipe(as, M.fold(R.getMonoid(monoid)))
+
+// given a function shaped `A` -> `B` -> `C`, a Reader<A> and a Reader<B>,
+// return a Reader<C>
+export const three = <R, A, B, C>(
+  f: (a: A) => (b: B) => C,
+  readerA: R.Reader<R, A>,
+  readerB: R.Reader<R, B>
+): R.Reader<R, C> => pipe(readerA, R.map(f), R.ap(readerB))
+
+type Env = {
+  translations: {
+    greeting: string
+    happyBirthday: string
+    genericPleasantry: string
+  }
+  date: Date
+}
+
+const thingOne = (name: string) => ({ translations }: Env) =>
+  `${translations.greeting}, ${name}!`
+
+const thingTwo = (birthday: Date) => ({ date, translations }: Env) =>
+  date.getMonth() == birthday.getMonth() &&
+  date.getDate() == birthday.getDate()
+    ? `${translations.happyBirthday}!`
+    : `${translations.genericPleasantry}.`
+
+// given the functions `thingOne` and `thingTwo`, combine them to make a
+// function that creates a birthday greeting.
+export const four = (
+  name: string,
+  birthday: Date
+): R.Reader<Env, string> =>
+  pipe(
+    A.sequenceT(R.reader)(thingOne(name), thingTwo(birthday)),
+    R.map(([a, b]) => `${a} ${b}`)
+  )