# N-ary Functors

## Using existing instances

`Functor`

and `Bifunctor`

are both in `base`

, but what about `Trifunctor`

? `Quadrifunctor`

? There must be a better solution than creating an infinite tower of typeclasses. Here's the API I managed to implement:

```
> nmap <#> (+1) <#> (+2) $ (0, 0)
(1,2)
> nmap <#> (+1) <#> (+2) <#> (+3) $ (0, 0, 0)
(1,2,3)
> nmap <#> (+1) <#> (+2) <#> (+3) <#> (+4) $ (0, 0, 0, 0)
(1,2,3,4)
```

What about `Contravariant`

and `Profunctor`

? No need to define `Bicontravariant`

nor `Noobfunctor`

, the `NFunctor`

typeclass supports contravariant type-parameters too!

```
> let intToInt = succ
> let intToString = nmap <#> show $ succ
> let stringToString = nmap >#< length <#> show $ succ
> intToInt 3
4
> intToString 3
"4"
> stringToString "foo"
"4"
```

As the examples above demonstrate, n-ary-functor has an equivalent for both the `Functor ((->) a)`

instance and the `Profunctor (->)`

instance. Even better: when writing your own instance, you only need to define an `NFunctor (->)`

instance, and the `NFunctor ((->) a)`

instance will be derived for you. `NFunctor ((->) a b)`

too, but that's less useful since that `nmap`

is just the identity function.

That's not all! Consider a type like `StateT s m a`

. The last type parameter is covariant, but what about the first two? Well, `s -> m (a, s)`

has both positive and negative occurences of `s`

, so you need both an `s -> t`

and a `t -> s`

function in order to turn a `StateT s m a`

into a `StateT t m a`

. And what about `m`

? You need a natural transformation `forall a. m a -> n a`

. Yes, n-ary-functor supports these too!

```
> let stateIntIdentityInt = ((`div` 2) <$> get) >>= lift . Identity
> let stateStringMaybeString = nmap
<#>/>#< (flip replicate '.', length) -- (s -> t, t -> s)
<##> NT (Just . runIdentity) -- NT (forall a. m a -> n a)
<#> show -- a -> b
$ stateIntIdentityInt
> runStateT stateIntIdentityInt 4
Identity (2,4)
> runStateT stateStringMaybeString "four"
Just ("2","....")
```

Notice how even in such a complicated case, no type annotations are needed, as n-ary-functor is written with type inference in mind.

## Defining your own instance

When defining an instance of `NFunctor`

, you need to specify the variance of every type parameter using a "variance stack" ending with `(->)`

. Here is the instance for `(,,)`

, whose three type parameters are covariant:

```
instance NFunctor (,,) where
type VarianceStack (,,) = CovariantT (CovariantT (CovariantT (->)))
nmap = CovariantT $ \f1
-> CovariantT $ \f2
-> CovariantT $ \f3
-> \(x1,x2,x3)
-> (f1 x1, f2 x2, f3 x3)
```

Its `nmap`

then receives 3 functions, which it applies to the 3 components of the 3-tuple.

Here is a more complicated instance, that of `StateT`

:

```
instance NFunctor StateT where
type VarianceStack StateT = InvariantT (Covariant1T (CovariantT (->)))
nmap = InvariantT $ \(f1, f1')
-> Covariant1T $ \f2
-> CovariantT $ \f3
-> \body
-> StateT $ \s'
-> fmap (f3 *** f1) $ unwrapNT f2 $ runStateT body $ f1' s'
```

The `s`

type parameter is "invariant", a standard but confusing name which does *not* mean that the parameter cannot vary, but rather that we need both an `s -> t`

and a `t -> s`

. The `m`

parameter is covariant, but for a type parameter of kind `* -> *`

, so we follow the convention and add a `1`

to the name of the variance transformer, hence `Covariant1T`

.

We've seen plenty of strange variances already and n-ary-functor provides stranger ones still (can you guess what the `ðŸ‘»#ðŸ‘»`

operator does?), but if your type parameters vary in an even more unusual way, you can define your own variance transformer. Here's what the definition of `CovariantT`

looks like:

```
newtype CovariantT to f g = CovariantT
{ (<#>) :: forall a b
. (a -> b)
-> f a `to` g b
}
```

One thing which is unusual in that newtype definition is that instead of naming the eliminator `unCovariantT`

, we give it the infix name `(<#>)`

. See this blog post for more details on that aspect.

Let's look at the type wrapped by the newtype. `to`

is the rest of the variance stack, so in the simplest case, `to`

is just `(->)`

, in which case the wrapped type is `(a -> b) -> f a -> g b`

, which is really close to the type of `fmap`

. The reason we produce a `g b`

instead of an `f b`

is because previous type parameters might already be mapped; for example, in `nmap <#> show <#> show $ (0, 0)`

, the overall transformation has type `(,) Int Int -> (,) String String`

, so from the point of view of the second `(<#>)`

, `f`

is `(,) Int`

and `g`

is `(,) String`

.

One last thing is that variance transformers must implement the `VarianceTransformer`

typeclass. It simply ensures that there exists a neutral argument, in this case `id`

, which doesn't change the type parameter at all.

```
instance VarianceTransformer CovariantT a where
t -#- () = t <#> id
```

### Flavor example

A concrete situation in which you'd want to define your own variance transformer is if you have a DataKind type parameter which corresponds to a number of other types via type families.

```
import qualified Data.ByteString as Strict
import qualified Data.ByteString.Lazy as Lazy
import qualified Data.Text as Strict
import qualified Data.Text.Lazy as Lazy
data Flavor
= Strict
| Lazy
type family ByteString (flavor :: Flavor) :: * where
ByteString 'Lazy = Lazy.ByteString
ByteString 'Strict = Strict.ByteString
type family Text (flavor :: Flavor) :: * where
Text 'Lazy = Lazy.Text
Text 'Strict = Strict.Text
data File (flavor :: Flavor) = File
{ name :: Text flavor
, size :: Int
, contents :: ByteString flavor
}
```

In order to convert a `File 'Lazy`

to a `File 'Strict`

, we need to map both the underlying `Text 'Lazy`

to a `Text 'Strict`

and the underlying `ByteString 'Lazy`

to a `ByteString 'Strict`

. So those are exactly the two functions our custom variance transformer will ask for:

```
newtype FlavorvariantT to f g = FlavorvariantT
{ (ðŸ˜‹#ðŸ˜‹) :: forall flavor1 flavor2
. ( ByteString flavor1 -> ByteString flavor2
, Text flavor1 -> Text flavor2
)
-> f flavor1 `to` g flavor2
}
instance VarianceTransformer FlavorvariantT a where
t -#- () = t ðŸ˜‹#ðŸ˜‹ (id, id)
```

We can now implement our `NFunctor File`

instance by specifying that its `flavor`

type parameter is flavorvariant.

```
instance NFunctor File where
type VarianceStack File = FlavorvariantT (->)
nmap = FlavorvariantT $ \(f, g)
-> \(File n s c)
-> File (g n) s (f c)
```