📝 Add explanations and hints for exo 3

This commit is contained in:
Hugo Saracino 2020-09-14 14:38:19 +02:00
parent c0ca5d234c
commit 7500ab98a0

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@ -5,10 +5,36 @@ import * as Option from 'fp-ts/lib/Option';
import { unimplemented } from '../utils'; import { unimplemented } from '../utils';
// Have you ever looked at the methods provided by `fp-ts` own `Array` and
// `ReadonlyArray` modules? They expose a load of functions to manipulate
// those collections.
//
// Some of those you likely are already familiar with, like `map` or `filter`.
// The difference with JavaScript's native `Array.prototype` methods is that
// these are more `fp-ts` friendly.
//
// In the following exercice, we will take a look at `Array.sort`. Contrary to
// its JavaScript counterpart, `fp-ts` sort takes as an argument something of
// type `Ord<T>` where `T` is the type of elements contained in the collection.
//
// You can think as `Ord<T>` as "something which describes how to order `T`s".
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// SORT PRIMITIVE TYPES // // SORT PRIMITIVE TYPES //
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// The next two functions only take an array of values of a primitive JS type
// like `string` or `number` and return a new array with those values but
// sorted.
//
// Obviously, we want to call `ReadonlyArray.sort` (the `fp-ts` version! no
// cheating). But, contrary to `ReadonlyArray.prototype.sort` which takes an
// ordering function, this sort will only accept an `Ord<T>`.
//
// HINT: The `Ord` module from `fp-ts` exposes some preconstructed instances
// of `Ord<T>` for a few primitive `T`s such as `ordString: Ord<string>` or
// `ordNumber: Ord<number>`.
export const sortStrings: ( export const sortStrings: (
strings: ReadonlyArray<string>, strings: ReadonlyArray<string>,
) => ReadonlyArray<string> = unimplemented; ) => ReadonlyArray<string> = unimplemented;
@ -21,6 +47,14 @@ export const sortNumbers: (
// REVERSE SORT // // REVERSE SORT //
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// This next function will sort an array of numbers but in descending order
// (which unfortunately is the reverse ordering from the provided `ordNumber`).
//
// Sure, we could just use `sortNumbers` defined earlier and then reverse the
// whole array but that would be horribly inefficient wouldn't it?
//
// HINT: Any ordering can be reversed with a simple function `Ord.getDualOrd`.
export const sortNumbersDescending: ( export const sortNumbersDescending: (
numbers: ReadonlyArray<number>, numbers: ReadonlyArray<number>,
) => ReadonlyArray<number> = unimplemented; ) => ReadonlyArray<number> = unimplemented;
@ -29,6 +63,16 @@ export const sortNumbersDescending: (
// SORT OPTIONAL VALUES // // SORT OPTIONAL VALUES //
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// This next function will sort an array of numbers wrapped in `Option` with
// the following constraint: `Option.none` < `Option.some(_)`.
//
// As such, we cannot simply use `ordNumber` because it has type `Ord<number>`
// and we need an instance of `Ord<Option<number>>`.
//
// HINT: Some of `fp-ts` wrapper types such as `Option` do already have a way
// of building an `Ord` instance for their qualified inner type. You may want
// to take a look at `Option.getOrd`.
export const sortOptionalNumbers: ( export const sortOptionalNumbers: (
optionalNumbers: ReadonlyArray<Option.Option<number>>, optionalNumbers: ReadonlyArray<Option.Option<number>>,
) => ReadonlyArray<Option.Option<number>> = unimplemented; ) => ReadonlyArray<Option.Option<number>> = unimplemented;
@ -37,6 +81,19 @@ export const sortOptionalNumbers: (
// SORT COMPLEX OBJECTS // // SORT COMPLEX OBJECTS //
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// Primitive types are nice and all, but sometimes you want to sort some values
// with many fields by considering only one of those fields with a more
// primitive type.
//
// In the next two functions, we start with an array of `Person`s which have a
// `name` and may have an `age`. One function will sort our array by alphabetic
// ordering of the person's names, and the other will sort it by the person's
// ages.
//
// HINT: You can build an instance of `Ord` specialized for a field for a
// record with many fields by declaring how to access that field and which
// primitive `Ord` instance to use. This can be achieved with `Ord.contramap`.
export interface Person { export interface Person {
readonly name: string; readonly name: string;
readonly age: Option.Option<number>; readonly age: Option.Option<number>;
@ -54,6 +111,8 @@ export const sortPersonsByAge: (
// COMBINE SORTING SCHEMES // // COMBINE SORTING SCHEMES //
/////////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////////
// TBD
export const sortPersonsByAgeThenByName: ( export const sortPersonsByAgeThenByName: (
person: ReadonlyArray<Person>, person: ReadonlyArray<Person>,
) => ReadonlyArray<Person> = unimplemented; ) => ReadonlyArray<Person> = unimplemented;