# Fantas, Eel, and Specification 18: Bifunctor + Profunctor

26 Jun 2017**The worst is behind us**. We’ve mastered the `Monad`

, conquered the `Comonad`

, and surmounted the `Semigroup`

. Consider these last two posts to be a **cool-down**, because **the end is in sight**. Today, to enjoy our first week of rest, we’re going to revise **functors**.
A number of times, we’ve seen types with **two** inner types: `Either`

, `Pair`

, `Task`

, `Function`

, and so on. However, in all cases, we’ve **only** been able to `map`

over the **right-hand** side. Well, Fantasists, the reason has something to do with a concept called **kinds** that we won’t go into. Instead, let’s look at **solutions**.

We’ll take a type like `Either`

or `Pair`

. These types have **two** inner types (*left* and *right*!) that we could `map`

over. The `Bifunctor`

class allows us to deal with both:

```
bimap :: Bifunctor f
=> f a c
~> (a -> b, c -> d)
-> f b d
```

It’s pretty much **exactly like Functor**, except we are mapping

**two at a time**! What does this look like in

*actual*code?

```
Either.prototype.bimap =
function (f, g) {
return this.cata({
Left: f,
Right: g
})
}
Pair.prototype.bimap =
function (f, g) {
return Pair(f(this._1),
g(this._2))
}
Task.prototype.bimap =
function (f, g) {
return Task((rej, res) =>
this.fork(e => rej(f(e)),
x => res(g(e))))
}
```

Hopefully, no huge surprises. We apply the **left function** to any mention of the **left value**, and the same for the right. Even the laws are just **doubled-up** versions of the `Functor`

laws:

```
// For some functor U:
// Identity
U.map(x => x) === U
// Composition
U.map(f).map(g) ===
U.map(x => g(f(x)))
// For some bifunctor C:
// Identity
C.bimap(x => x, x => x) === C
// Composition
C.bimap(f, g).bimap(h, i) ===
C.bimap(l => h(f(l)),
r => i(g(r)))
```

A lot of brackets, but look closely: the laws are the same, but we have **two independent “channels”** on the go!

Fun fact: if you have a`Bifunctor`

instance for some type`T`

, you can automatically derive a fully-lawful`Functor`

instance for`T a`

with`f => bimap(x => x, f)`

.Hooray, Free upgrades!

So, is this useful? **Yes**! Let’s look at a neat little example using `Either`

for a second:

```
//- Where everything changes...
const login = user =>
isValid(user) ? Right(user)
: Left('Boo')
//- Function map === "piping".
//+ failureStream :: String
//+ -> HTML
const failureStream =
(x => x.toUpperCase())
.map(x => x + '!')
.map(x => '<em>' + x + '</em>')
//+ successStream :: User
//+ -> HTML
const successStream =
(x => x.name)
.map(x => 'Hey, ' + x + '!')
.map(x => '<h1>' + x + '</h1>')
//- We can now pass in our two
//- possible application flows
//- using `bimap`!
login(user).bimap(
failureStream,
successStream)
```

Note that this would also work with `Task`

! We’re in a situation where we want to transform a potential success *or* failure, and `bimap`

lets us supply both at once. *Cool*, right? **Straightforward**, too!

Now, `Function`

is a *slightly* different story. Effectively, we can `contramap`

over its **left-hand type** (the **input**) and `map`

over its **right-hand type** (the **output**). It turns out there’s a fancy name for this sort of thing, too: `Profunctor`

.

```
promap :: Profunctor p
=> p b c
~> (b -> a, c -> d)
-> p a d
```

You can think of it as adding a **before** and **after** step to some process. Naturally, the laws look like a **mooshmash** of `Contravariant`

and `Functor`

:

```
// For some profunctor p:
// Identity...
P.promap(a => a, b => b) === P
// Composition...
p.promap(f, i).promap(g, h) ===
p.promap(a => f(g(a)),
b => h(i(b)))
```

Guess what? You can build a functor`P a`

out of any profunctor`P`

:`f => promap(x => x, f)`

is all it takes. So manyfree upgrades!

The **left-hand** side looks like `Contravariant`

, and the **right-hand** side like `Functor`

. Of course, we’ve seen a `Profunctor`

already: ** Function**! However, to give a slightly more

**exciting**example, let’s look at one of my

**favourites**:

`Costar`

.```
//- Fancy wrapping around a specific type
//- of function: an "f a" to a "b"
//+ Costar f a b = f a -> b
const Costar = daggy.tagged('run')
//- Contramap with the "before" function,
//- fold, then apply the "after" function.
//+ promap :: Functor f
//+ => Costar f b c
//+ -> (b -> a, c -> d)
//+ -> Costar f a d
Costar.prototype.promap = function (f, g) {
return Costar(x => g(this.run(x.map(f))))
}
//- Takes a list of ints to the sum
//+ sum :: Costar Array Int Int
const sum = Costar(xs =>
xs.reduce((x, y) => x + y, 0))
//- Make every element 1, then sum them!
const length = sum.promap(_ => 1, x => x)
//- Is the result over 5?
const isOk = sum.promap(x => x, x => x > 5)
//- Why not both? Is the length over 5?
const longEnough = sum.promap(
_ => 1, x => x > 5)
// Returns false!
longEnough.run([1, 3, 5, 7])
```

`Costar`

allows us to take the idea of `reduce`

and wrap it in a `Profunctor`

. We can use `promap`

to **prepare** different inputs for the reduction *and* **manipulate** the result. You may have heard of this idea before: **map/reduce**. No matter how **complex** the process, there’s a good chance that you can express it in a `Profunctor`

!

These are, of course, *very* quick overviews of `Bifunctor`

and `Profunctor`

. That said, if you’re comfortable now with `Functor`

and you remember the post on `Contravariant`

, there’s **nothing new** to learn! We’re really just building on ideas we’ve already had. `Bifunctor`

might seem a little *underwhelming* now that we’ve seen the **power** of `Monad`

and `Comonad`

, but it’s *twice* as powerful as `Functor`

: we can define a flow for **success** and **error**, for **left** and **right**, for… well, any **two things**!

As for `Profunctor`

, it’s a pretty massive topic once you start digging. `Costar`

is the opposite of `Star`

, which is an `a -> f b`

function; why not think about how to implement that? Would you need any **special conditions** to make it a `Profunctor`

?

Take the article’s gist, and `bimap`

and `promap`

until the cows come home, Fantasists, for there is only **one** article left: `Semigroupoid`

and `Category`

. Expect **high drama**, **hard maths**, and **herds of monoids**. Well, maybe not the second thing…

Until then!

♥