FUNCTORS, APPLICATIVES AND MONADS IN HASKELL
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Disclaimer
In this article we will learn about the three main abstract structures in Haskell: functors, applicatives and monads. Before getting started I want to make a disclaimer:
this guide is not intended to replace the existing literature about monads in Haskell, this is just the method I have used to understand them. I can't guarantee you that reading this post will give you the right insight to understand these topics, I think it's impossible to understand them on the first try, even beginners tutorials struggle on that. Furthermore, keep in mind that you may have a different skill set from mine; before learning about monads I already had a background in category theory and in functional programming, this guide is based on this assumption.
Before talking about functors, applicative and monads, we need to figure out a couple of things about Haskell's type system.
Data Types
As you might already know, Haskell provides a lot of builtin data types, such as
Int
, Integer
,
Char
, String
,
Maybe
, etc.
In order to implement a new type, we can use either
data
,
newtype
or type
.
Let us break down the differences:
Algebraic Data Type
An algebraic data type is a type formed by combining two or more types. In Haskell, they are defined using the
data
keyword. An ADT allows us to define a new data type with an
arbitrary number of constructors, each constructor can have an arbitrary
number of arguments. For example, Haskell defines the
Bool
type as
data Bool = False  True
This is read as “Boolean is either False or True”.
The word “algebraic” refers to the fact that these kind of data types are built using algebraic operators such as
and
(product) and
or
(sum). They work in the following way:

sum is equivalent to the logical (exclusive)disjunction(e.g.,
A  B
means either A or B); 
product is equivalent to the logical conjunction(e.g.,
A B
means both A and B).
data Tree t = Leaf t  Node (Tree t) (Tree t) deriving Show
as you can see, a tree can either be a leaf(i.e. the end node) or a node with two subtrees.
The last part(deriving Show
) allows us to print out a
Tree
object in the console:
Prelude> t = Node (Leaf 0) (Node (Leaf 1) (Leaf 2))
Prelude> t
Node (Leaf 0) (Node (Leaf 1) (Leaf 2))
Newtype
Another way to declare new data types is by using the
newtype
keyword.
This type declaration can be used whether you want to declare a new type that has exactly one
constructor, The advantage of using
newtype
over
data
is that the
former is optimizer by the compiler, hence it is faster than the latter.
Type
The last way to declare data types in Haskell is by using
type
keyword. This declaration consists in a
type synonymous to another data type. For example, the
String
type is defined as
type String = [Char]
type
keyword works like the
typedef
keyword from C/C++.
Typeclasses
Now that we have a brief understanding of how types works in Haskell, we can introduce another core topic: typeclasses.
A typeclass is a construct that implements adhoc polymorphism(i.e., overloading). A typeclass consists in a set of functions that defines a certain behavior; an instance of a typeclass implements that behavior concretely.
Typeclasses are quite similar to interfaces from the objectoriented world: defining a sort of blueprint and then delegating the concrete classes to implement the methods(i.e., the behavior). Be sure, however, to not mix up OOP classes and Haskell classes, because they are not the same thing.
For example, the
Eq
typeclass is defined as follows:
class Eq a where
(==) :: a > a > Bool
(/=) :: a > a > Bool
which contains all types that allow equality.
Functor
In order to introduce the concept of functors, let first see the definition of the
Maybe
data type:
data Maybe a = Just a  Nothing
Now suppose that we want to apply the function
 Increment by one
inc :: Int > Int
inc n = n + 1
to the wrapped value
Just 2
. If we try to do this, we get this error:
*Main> inc (Just 2)
<interactive>:2:6: error:
• Couldn't match expected type 'Int' with actual type 'Maybe a0'
• In the first argument of 'inc', namely '(Just 2)'
In the expression: inc (Just 2)
In an equation for 'it': it = inc (Just 2)
This is where functors come in.
If we try to apply the>A functor is a typeclass defined as follows:A functor typeclass defines theclass Functor f where fmap :: (a > b) > f a > f b
fmap
function,fmap
applies a function(a > b
) to a wrapped value (f a
) and returns a new wrapped value (f b
).
Inc
function to the wrapped value
Just 2
using
fmap
we should get the wrapped value
Just 3
, let us try:
*Main> fmap inc (Just 2)
Just 3
*Main> inc <$> (Just 2)  Same result, using the infix operator
Just 3
That is what we were expecting. Let us now see the instantiation of the
Functor
classtype for the
Maybe
datatype:
instance Functor Maybe where
fmap f (Just a) = Just (f a)
fmap f Nothing = Nothing
Let us break this down:

In the first case, we “extract” the value
a
from theJust
, we then apply thef
function toa
and we finally wrap the result value in a newJust
context; 
In the second case, we just return a
Nothing
.
Increment a Treelike Data Structure
Let us now to solve this simple computer science problem. Given the following treelike data structure increment each node by 1. The first thing we want to do is to give an internal representation of the binary tree in Haskell using an algebraic data type:
data Tree t = Leaf t  Node t (Tree t) (Tree t) deriving Show
In order to work with this
Tree
type,
we can create a new instance of the
Functor
classtype:
instance Functor Tree where
fmap f (Leaf t) = Leaf (f t)
fmap f (Node t leftSub rightSub) = Node (f t) (f <$> leftSub) (f <$> rightSub)
Again, this is what happens here:

If the node is a leaf, apply the function
f
and wrap it in a newLeaf
; 
If the node is internal, first apply the
f
function to the node's state, then apply thefmap
function to the left and right subtree and finally wrap the result in a newNode
.
*Main> let t = (Node 0 (Leaf 1) (Node 2 (Leaf 3) (Leaf 4)))
*Main> t
Node 0 (Leaf 1) (Node 2 (Leaf 3) (Leaf 4))
Then we can apply the
inc
function to the
t
tree:
*Main> fmap inc t
Node 1 (Leaf 2) (Node 3 (Leaf 4) (Leaf 5))
Applicative Functors
Applicatives extend the notion of functors. An applicative allows us to apply a wrapped function to a wrapped value. Here's an example:
Suppose that we want to apply the wrapped function
Just (+1)
to
Just 1
,
functors are not enough for this kind of operation:
Prelude> Just (+1) <$> Just 2
<interactive>:1:1: error:
• Couldn't match expected type 'a1 > b'
with actual type 'Maybe (a0 > a0)''
• Possible cause: 'Just' is applied to too many arguments
In the first argument of '(<$>)', namely 'Just (+ 1)''
In the expression: Just (+ 1) <$> Just 2
In an equation for 'it': it = Just (+ 1) <$> Just 2
• Relevant bindings include
it :: Maybe b (bound at <interactive>:1:1)
In order to overcome this error, we can use the applicative operator
(<*>
):
Prelude> Just (+1) <*> Just 2
Just 3
Let us give a proper definition.
You now may be wondering: “what is the point of applying a wrapped function? This would have produced the exact same result if we had used a functor.” The answer is, both functors and applicatives are not limited to the>An applicative is a typeclass defined as followsAn applicative applies a wrapped function(class Functor f => Applicative f where pure :: a > f a <*> :: f (a > b) > f a > f b
f (a > b)
) to a wrapped value(f a
) and returns a new wrapped value(f b
).
Maybe
datatype,
in fact we can use these abstract structures with any kind of “contexts”.
Let us see a more concise example:
Suppose that we want to execute two different functions to the
[5, 4, 3, 2, 1]
array, in particular these two functions
will do the following:
 Decrement by one
dec :: Int > Int
dec n = n  1
and
 Multiply by two
mul :: Int > Int
mul n = n * 2
In order to apply the first “context”(i.e., the two functions) to
the other(i.e., the array) we can use an applicative:
*Main> let arr = [dec, mul] <*> [5, 4, 3, 2, 1]
*Main> arr
[4,3,2,1,0,10,8,6,4,2]
The resulting array is a vector with twice the size of the original.
Just like for the previous section let us see the instantiation of the
Applicative
classtype for the
Maybe
datatype:
instance Functor Maybe where
fmap f (Just a) = Just (f a)
fmap f Nothing = Nothing
instance Applicative Maybe where
pure = Just
Just f <*> Just x = Just(f x)
_ <*> _ = Nothing
Monads
In this last section we will merge everything we have learned so far in this tutorial to understand the concept of monads. A monad extends the notion of applicatives by allowing us to apply a function that returns a wrapped value to a wrapped value. This sounds confusing and intimidating, let us see an example.
Consider the following function:
 Returns n/2 if and only if 'n' is an even integer
half :: Integral a => a > Maybe a
half n = if even n
then Just (n `div` 2)
else Nothing
It takes a number and, if it is even, it returns half of its size in a wrapped value,
otherwise it returns
Nothing
. Since it returns a
Maybe
, a first approach
could be to just use a functor, let us try:
*Main> half <$> (Just 2)
Just (Just 1)
We get a wrapped value…wrapped in another wrapper. The problem with this solution is that
the functor does not support this kind of functions(i.e., a function that returns a wrapped value),
so we need something more advanced, we need a monad:
*Main> (Just 2) >>= half
Just 1
The monad operator(»=, pronounced bind), allows us to do that. Here a real definition:
We can also create a stream of monads by chaining the bind operator:>A monad is a typeclass defined as follows:A monad applies a function that returns a wrapped value (class Applicative m => Monad m where (>>=) :: m a > (a > m b) > m b (>>) :: m a > m b > m b return :: a > m a
a > m b
) to a wrapped value(m a
) and then returns a new wrapped value(m b
). TheMaybe
datatype is also a monad:instance Functor Maybe where fmap f (Just a) = Just (f a) fmap f Nothing = Nothing instance Applicative Maybe where pure = Just Just f <*> Just x = Just(f x) _ <*> _ = Nothing instance Monad Maybe where Nothing = Nothing Just x >>= f = f x
*Main> (Just 40) >>= half >>= half >>= half
Just 5
This tells us that
Maybe
is both a functor, an applicative and a monad.
IO Monad
One of the most useful monads in Haskell is the IO monad. This monad deals with the unpure, input/output world. It defines three functions:
getLine :: IO String  no arguments, read user input
readFile :: FilePath > IO String  takes the filepath, returns filepath's content
putStrLn :: String > IO ()  takes a string and print it
Let us see them in action. In your working directory create a file with some text inside:
$> cat foo
1. Hello World
2. This is the second line
3. This is the last line
The following code will ask the user for a file path, then will try to
read the file and finally will print out its content:
Prelude> putStr "Enter the file path: " >> getLine >>= readFile >>= putStrLn
Enter the file path: foo
1. Hello World
2. This is the second line
3. This is the last line
We can improve this chain of operations by using the
DO notation(it is just a syntax sugar for chains of monads):
main :: IO ()
main = do
putStr "Enter the file path: "
file < getLine
buf < readFile file
let content = "File content:\n" ++ buf
putStrLn content
Which produce the same result:
*Main> main
Enter the file path: foo
File content:
1. Hello World
2. This is the second line
3. This is the last line