# # Applicative Functor

## # Alternative definition

Since every Applicative Functor is a Functor, `fmap`

can always be used on it; thus the essence of Applicative is the pairing of carried contents, as well as the ability to create it:

```
class Functor f => PairingFunctor f where
funit :: f () -- create a context, carrying nothing of import
fpair :: (f a,f b) -> f (a,b) -- collapse a pair of contexts into a pair-carrying context
```

This class is isomorphic to `Applicative`

.

```
pure a = const a <$> funit = a <$ funit
fa <*> fb = (\(a,b) -> a b) <$> fpair (fa, fb) = uncurry ($) <$> fpair (fa, fb)
```

Conversely,

```
funit = pure ()
fpair (fa, fb) = (,) <$> fa <*> fb
```

## # Common instances of Applicative

### # Maybe

`Maybe`

is an applicative functor containing a possibly-absent value.

```
instance Applicative Maybe where
pure = Just
Just f <*> Just x = Just $ f x
_ <*> _ = Nothing
```

`pure`

lifts the given value into `Maybe`

by applying `Just`

to it. The `(<*>)`

function applies a function wrapped in a `Maybe`

to a value in a `Maybe`

. If both the function and the value are present (constructed with `Just`

), the function is applied to the value and the wrapped result is returned. If either is missing, the computation can't proceed and `Nothing`

is returned instead.

### # Lists

One way for lists to fit the type signature `<*> :: [a -> b] -> [a] -> [b]`

is to take the two lists' Cartesian product, pairing up each element of the first list with each element of the second one:

```
fs <*> xs = [f x | f <- fs, x <- xs]
-- = do { f <- fs; x <- xs; return (f x) }
pure x = [x]
```

This is usually interpreted as emulating nondeterminism, with a list of values standing for a nondeterministic value whose possible values range over that list; so a combination of two nondeterministic values ranges over all possible combinations of the values in the two lists:

```
ghci> [(+1),(+2)] <*> [3,30,300]
[4,31,301,5,32,302]
```

### # Infinite streams and zip-lists

There's a class of `Applicative`

s which "zip" their two inputs together. One simple example is that of infinite streams:

```
data Stream a = Stream { headS :: a, tailS :: Stream a }
```

`Stream`

's `Applicative`

instance applies a stream of functions to a stream of arguments point-wise, pairing up the values in the two streams by position. `pure`

returns a constant stream – an infinite list of a single fixed value:

```
instance Applicative Stream where
pure x = let s = Stream x s in s
Stream f fs <*> Stream x xs = Stream (f x) (fs <*> xs)
```

Lists too admit a "zippy" `Applicative`

instance, for which there exists the `ZipList`

newtype:

```
newtype ZipList a = ZipList { getZipList :: [a] }
instance Applicative ZipList where
ZipList xs <*> ZipList ys = ZipList $ zipWith ($) xs ys
```

Since `zip`

trims its result according to the shortest input, the only implementation of `pure`

that satisfies the `Applicative`

laws is one which returns an infinite list:

```
pure a = ZipList (repeat a) -- ZipList (fix (a:)) = ZipList [a,a,a,a,...
```

For example:

```
ghci> getZipList $ ZipList [(+1),(+2)] <*> ZipList [3,30,300]
[4,32]
```

The two possibilities remind us of the outer and the inner product, similar to multiplying a 1-column (`n x 1`

) matrix with a 1-row (`1 x m`

) one in the first case, getting the `n x m`

matrix as a result (but flattened); or multiplying a 1-row and a 1-column matrices (but without the summing up) in the second case.

### # Functions

When specialised to functions `(->) r`

, the type signatures of `pure`

and `<*>`

match those of the `K`

and `S`

combinators, respectively:

```
pure :: a -> (r -> a)
<*> :: (r -> (a -> b)) -> (r -> a) -> (r -> b)
```

`pure`

must be `const`

, and `<*>`

takes a pair of functions and applies them each to a fixed argument, applying the two results:

```
instance Applicative ((->) r) where
pure = const
f <*> g = \x -> f x (g x)
```

Functions are the prototypical "zippy" applicative. For example, since infinite streams are isomorphic to `(->) Nat`

, ...

```
-- | Index into a stream
to :: Stream a -> (Nat -> a)
to (Stream x xs) Zero = x
to (Stream x xs) (Suc n) = to xs n
-- | List all the return values of the function in order
from :: (Nat -> a) -> Stream a
from f = from' Zero
where from' n = Stream (f n) (from' (Suc n))
```

... representing streams in a higher-order way produces the zippy `Applicative`

instance automatically.

#### # Remarks

### # Definition

```
class Functor f => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
```

Note the `Functor`

constraint on `f`

. The `pure`

function returns its argument embedded in the `Applicative`

structure. The infix function `<*>`

(pronounced "apply") is very similar to `fmap`

except with the function embedded in the `Applicative`

structure.

A correct instance of `Applicative`

should satisfy the **applicative laws**, though these are not enforced by the compiler:

```
pure id <*> a = a -- identity
pure (.) <*> a <*> b <*> c = a <*> (b <*> c) -- composition
pure f <*> pure a = pure (f a) -- homomorphism
a <*> pure b = pure ($ b) <*> a -- interchange
```