# Fantas, Eel, and Specification 10: Alt, Plus, and Alternative

24 Apr 2017We’re in **double digits**! Isn’t this exciting? It also means that, by my estimations, we’re **well over half way**! Before we get *too* excited by `Profunctor`

and `Comonad`

, though, might I tempt you with an… `Alternative`

?

Today, we’re going to bundle together **three** very well-related entries in the spec, starting with `Alt`

. This little typeclass has one function:

```
alt :: Alt f => f a ~> f a -> f a
```

As we know, with great algebraic structure, come great **laws**:

```
// Associativity
a.alt(b).alt(c) === a.alt(b.alt(c))
// Distributivity
a.alt(b).map(f) === a.map(f).alt(b.map(f))
```

The **associativity** law is *exactly* what we saw in the `Semigroup`

post with `concat`

! As we said back then, think of it as, “*Keeping left-to-right order the same, you can combine the elements however you like*”.

**Distributivity** gives us another clue that we might be looking at something a bit `Semigroup`

-flavoured. We can ** map first** over the elements and then

`alt`

**or**

**and then**

`alt`

first`map`

over the result, and we’ll end up at the **same value**. Either way, some kind of “combination” definitely seems to be going on here.

So, **what is it**? Well… it’s like a **semigroup for functors**. It’s a way of combining values of a functor type *without* the requirement that the inner type be a `Semigroup`

. *Why*, you ask? Well, we get a little hint from the name.

`Alt`

allows us to provide some *alternative* value as a “fallback” when the first “fails”. Of course, this is particularly relevant to types with some notion of **failure**:

```
Maybe.prototype.alt = function (that) {
this.cata({
Just: _ => this,
Nothing: that
})
}
```

If we have a `Just`

on the left, we return it. Otherwise, we **fall back** to the second value! Naturally, you can chain as many of these as you like:

```
// Just(3) - note the "Nothing"s are
// usually the result of some functions.
Nothing.alt(Nothing).alt(Just(3))
```

It turns out there are **loads** of use cases for `alt`

, which isn’t too surprising if you look at it as a **functor-level if/else**. You can do database connection failover, API/resource routing, and, most magically of all, text parsing.

*Those last two were in PureScript and Haskell respectively, but don’t worry: in these languages,*

`alt`

has an operator, written as `<|>`

.The key thing all these cases have in common is that you want to *try* something with a contingency plan for *failure*. That’s all there is to `Alt`

!

If `Alt`

will be our functor-level `Semigroup`

, what’s our **functor-level Monoid**? In comes

`Plus`

, which extends `Alt`

with one more function called `zero`

:```
zero :: Plus f => () -> f a
```

Looks a bit like `Monoid`

’s `empty`

, right? Note that there’s no restriction on the `a`

, so this `zero`

value must work for **any type**. This one has **three laws**, but the first two will look really familiar to readers of the `Monoid`

post:

```
// Right identity - zero on the right
x.alt(A.zero()) === x
// Guess what this one's called?
A.zero().alt(x) === x
// The new one: annihilation
A.zero().map(f) === A.zero()
```

The left and right **identity** laws just say, “* zero makes no difference to the other value, regardless of which side of alt you put it*”.

**Annihilation**gives us a stronger idea of what

`zero`

does: **nothing**!

`Plus`

types *must*be functors; for a

`map`

call to do *nothing*in all cases, the type must have the ability to be

**empty**, whatever that means.

Think of our `Maybe`

type: what can we `map`

over with *any* function and not change the value? `Nothing`

! In fact, `() => Nothing`

is the **only valid** implementation of `zero`

for `Maybe`

.

What about `Array`

? Well, `map`

transforms every value in the array, so the only array that *wouldn’t* be changed is the empty one. `() => []`

is the **only valid** implementation of `zero`

for `Array`

.

We didn’t cover

`Array`

as an`Alt`

because it’s a bit of a funny one. Back when we discussed functors, we saw that`Array`

extendsour language to allow us to representseveral valuesat once. This can be thought of asnon-determinismif we see an`Array`

as the set ofpossible values. Thus, the`alt`

implementation for`Array`

is the same as`concat`

- all we’re doing is combining the two sets of possibilities!

So, `Plus`

adds to `Alt`

what `Monoid`

adds to `Semigroup`

, and, in fact, what `Applicative`

adds to `Apply`

: an **identity**. Are we bored of this pattern yet? I hope not, because we’re *still* not done with it! Incidentally, we can write custom `Semigroup`

and `Monoid`

types to encapsulate this behaviour so we can reuse the functions we talked about in their posts:

```
// The value MUST be an Alt-implementer.
const Alt = daggy.tagged('value')
// Alt is a valid semigroup!
Alt.prototype.concat = function (that) {
return Alt(this.value.alt(that.value))
}
// The value MUST be a Plus-implementer.
// And, as usual, we need a TypeRep...
const Plus = T => {
const Plus_ = daggy.tagged('value')
// Plus is a valid semigroup...
Plus_.prototype.concat =
function (that) {
return Plus(
this.value.alt(
that.value))
}
// ... and a valid monoid!
Plus_.empty = () => Plus_(T.zero())
}
```

Monoids are **everywhere**, I tell you. Stare at something long enough and it’ll start to look like a monoid.

The final boss level on this `Alt`

quest is `Alternative`

. There are **no special functions** for this one, as it is simply the name for a structure that implements both `Plus`

*and* `Applicative`

. Still, I know how much you *love* laws:

```
// Distributivity
x.ap(f.alt(g)) === x.ap(f).alt(x.ap(g))
// Annihilation
x.ap(A.zero()) === A.zero()
```

**Distributivity** is exactly as the same law that we saw with `Alt`

and `map`

at the beginning of all this, but now for `ap`

. We can either ** alt first** and

*then*

`ap`

the result to `x`

, **or**we can

**to both separately, and then**

`ap`

first`alt`

. Either way, we end up in the same place.**Annihilation** is a *really* scary word for a not-so-scary idea, if you think back to the `zero`

values we discussed earlier. You couldn’t apply a value to `Nothing`

, right? Or an empty list of functions? The **annihilation** law defines this behaviour: if you try to do **something with nothing**, you get **nothing**. Whatever you were doing is considered a *failure*, and `zero`

is returned.

You’ll often hear `Alternative`

types described as **monoid-shaped applicatives**, and this is a good intuition. We talked about `of`

as being the **identity** of `Applicative`

, but this is only at **context-level**. For an `Alternative`

type, `zero`

is the identity value at context- **and** value-level.

`Maybe`

, `Array`

, `Task`

, `Either`

: we’ve seen a lot of types that can very naturally implement `Alternative`

. You could even make `Function`

an `Alternative`

if you knew the output would be of a `Plus`

-implementing type. With that, you could then write a function whose body can do **extra computation** depending on the result; who needs `if/else`

?

That’s about all there is to it! `Alt`

, `Plus`

, and `Alternative`

are **under-appreciated** typeclasses, particularly within functional JavaScript. Take some time to look through your code, glare at the `if/else`

, `try/catch`

, and `switch`

blocks, and see whether they’re really just `alt`

s in disguise!

Next time, we’ll be looking into your **new favourite** typeclasses: `Foldable`

and `Traversable`

. Try to contain your excitement until then!

Take care ♥