What is functional Programming?

• Where it all started?

  • Lambda Calculus
    • 3 things in lambda calculus
    • Variables(x,y,z)
    • Building a function(λ notation)
    • Applying functions (λx.x+1) 5 5+1 = 6
  • Function in mathematics
    • A function is an expression that assigns each element of a set, call the domain, to exactly one element of a second set, call the codomain.
    • For Given input 1, at any given point, output is 1 for the function f(x)=x^2
  • functional programming is all about building functions for immutable variables. it is about writing declaratively with inputs and outputs without side effects.

• Immutability

  • An immutable data is the data that can’t be changed after creating
  • Avoids Race Condition
  • Avoid Side-effects.
  • when functions act on values, a new value is created and returned, rather than modifying the existing value.

• Pure and impure functions

• A pure function is a function which:
  • Given the same input, will always return the same output.
  • No side-effects.
  • Maintains no internal state.
  • No dependency on any external factor other than the input provided.
  • Referentially Transparent.
• Impure Function
  • No consistent output for a given input.
  • Has side effects
  • Uses value from out of scope of function.
  • An impure function inside a function makes the function impure.
• What is a Side Effect? * A side effect is any application state change that is observable outside the called function other than its return value. * Modifying any external variable(e.g., a global variable, or a variable in the parent function scope chain) * Logging to the console. * Writing to the screen. * Writing to a file. * Writing to a network. * Triggering any external process. * Eg. datetime, now, randomness.
  • Advantages of Pure Functions
    • Easy to reason out the code - Readability
    • Referentially Transparent.
      • the ability to replace an expression with it’s calculated value is called referential transparency.
      • eg let’s say f(x) - x*x, f(2)=4. f(2) could be used in places where we use 4 and vice versa.
      • Memoization and Caching
      • Parallel Processing
      • Easy to debug and Easy to test
      • Reusability

• Declarative and Imperative

• Imperative Programming
  • A style of programming which describes how the operations are performed by a computer.
  • Step by Step Fashion
  • define a numbers list and an empty odd_numbers list.
  • Loop over each number.
  • Check if number divisible by 2
  • if yes, append to the odd_numbers list.
  • if no, skip the number.
• Declarative Programming:
  • Explains what to be done.
  • Doesn’t explicitly state how the work will be done.
  • E.g. Filter the odd numbers from the list.

• Higher Order Functions / First Class Functions

• In FP, functions are first class citizens.
• Functions can be treated as values.
• They can be
  • Assigned as values,
  • Passed into functions, and
  • Returned from functions.
• A higher order function is a function that takes a function as an argument, or returns a function.
• Map, Filter and Reduce(Higher order functions)

• Advantages and Disadvantages of FP Advantages:

  • Functional programming enables and encourages a more abstract way of solving a problem. A more “mathematical” programming way
  • Code is easier to reason - Predictable code.
  • Greater Productivity - Less Boilerplate code.
  • Greater modularity and reuse.
  • Easier Testing.
  • Easier Debugging.
  • Future Ready
    • Parallel Programming
    • Asynchronous Programming Limitations:
  • Real world problems need Side Effects e.g) I/O Operations.
  • Immutability and Recursion could potentially lead to performance problems - Increased RAM usage.
  • Working with Immutable types - Recursion, Map, reduce, filter will be harder for people from imperative background
  • Lack of Tools

• Jargons to explore on your own

  • Promises
  • Asynchronous Functions
  • Concurrent Programming
  • Business Examples.
  • Currying
  • Lazy Evaluation.