feat(maybe): complete instance of applicative

[williamareynolds]

In order to complete the instance of Applicative, a method "apply" must
be added to the type. Technically, a Monad does not require an instance
of Applicative, but most modern functional languages and libraries
enforce this constraint due to the benefits derived when implementing
instances of other Categories. Particularly in the case of Maybe, this
instance is desirable due to the behavior of Maybes which wrap a
function. Because applying an undefined function to a value results in
an error, rather than undefined or null, mapping a function wrapped by
Maybe demands that the function be unwrapped and coerced to some
default before it can be used, or checking to see if it is defined. The
"apply" method allows a value wrapped by Maybe to be applied to a
function wrapped by Maybe.

Example:

import { maybe, IMaybe } from 'typescript-monads'

declare f(): string | undefined
declare g(): string | undefined

const fileName = maybe(f())
const mirrorUrl = maybe(g())
const makeUrl = (fileName: string) => (url: string) => `${url}/${name}`

/*
 * Without Applicative, we wind up nesting maps
 */
const getMirror: IMaybe<(url: string) => string> = fileName.map(name => makeUrl(name))

const mirror: IMaybe<string> = mirrorUrl.flatMap(url => getMirror.map(f => f(url)))

/*
 * With Applicative, we can directly apply a potentially undefined function
 */
const getMirror: IMaybe<(url: string) => string> = fileName.map(name => makeUrl(name))

const mirror: IMaybe<string> = fileName.apply(getMirror)

[patrickmichalina]

Dope, thank you!

[patrickmichalina]

Try updating the branch with master and see if a build will work now - I changed the circle-ci settings.

[patrickmichalina]

🎉 This PR is included in version 3.13.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀