What is functional programming?

What is functional programming?

Functional programming is a programming paradigm — a style of building the structure and elements of computer programs — that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data — Wikipedia

1.Pure functions

Here is a very strict definition of purity:

  • It returns the same result if given the same arguments (it is also referred as deterministic)
    • It should NOT uses a global object that was not passed as a parameter to the function.
    • It should NOT reads external files
    • Any function that relies on a random number generator cannot be pure.
  • It does not cause any observable side effects. Examples of observable side effects include modifying a global object or a parameter passed by reference.

Pure functions benefits

  • Given a parameter A → expect the function to return value B
  • Given a parameter C → expect the function to return value D

2. Immutability

When data is immutable, its state cannot change after it’s created. If you want to change an immutable object, you can’t. Instead, you create a new object with the new value.

3. Referential transparency

Basically, if a function consistently yields the same result for the same input, it is referentially transparent.

pure functions + immutable data = referential transparency

With this concept, a cool thing we can do is to memoize the function.

4. Functions as first-class entities

The idea of functions as first-class entities is that functions are also treated as values and used as data. Functions as first-class entities can:

  • refer to it from constants and variables
  • pass it as a parameter to other functions
  • return it as result from other functions

The idea is to treat functions as values and pass functions like data. This way we can combine different functions to create new functions with new behavior.

5.Higher-order functions

When we talk about higher-order functions, we mean a function that either:

  • takes one or more functions as arguments, or
  • returns a function as its result The doubleOperator function we implemented above is a higher-order function because it takes an operator function as an argument and uses it. You’ve probably already heard about filter, map, and reduce. Let’s take a look at these.

6.Filter

Given a collection, we want to filter by an attribute. The filter function expects a true or false value to determine if the element should or should not be included in the result collection. Basically, if the callback expression is true, the filter function will include the element in the result collection. Otherwise, it will not.

7. Map

The idea of map is to transform a collection.

The map method transforms a collection by applying a function to all of its elements and building a new collection from the returned values.

Let’s get the same people collection above. We don’t want to filter by “over age” now. We just want a list of strings, something like TK is 26 years old. So the final string might be :name is :age years old where :name and :age are attributes from each element in the people collection.

8. Reduce

The idea of reduce is to receive a function and a collection, and return a value created by combining the items.