is-maybe-a-functor.js

// is maybe a functor if it doesn't try-catch?

// Functor law (in haskell):
fmap id = id
fmap (p . q) = (fmap p) . (fmap q)

// in js:
func.map(_.identity) === _.identity(func)
func.map(_.compose(a, b)) === func.map(b).map(a)

// consider this, though:
var unpackX = function(o){ return o.x }
var unpackY = function(o){ return o.y }

var unpackXY = _.compose(unpackY, unpackX)
unpackXY({ x: { y: 3 } }) //= 3
unpackXY('foo') //= error: undefined is not an object

// Maybe maps over an object unless it holds `null` or `undefined` - the js 'nothing' values.
// in this case, there is no try-catch gaurds in map.
Maybe('foo').map(unpackXY) //= error - undefined is not an object
Maybe('foo').map(unpackX).map(unpackY) //= no error - undefined causes a skip.

Maybe is an abstract supertype of the types Just and Nothing and as such maybe is not a functor.

Just('foo').map(unpackXY) === Just('foo').map(unpackX).map(unpackY)
Nothing('foo').map(unpackXY) === Nothing('foo').map(unpackX).map(unpackY)

Both Just and Nothing are functors

The way you normally use Maybe is with a method like fromNullable which returns either a Just or Nothing
meaning you need to chain not map over it in order to do these multiple step procedures.