Gain insights into Haskell's functional programming by learning to write pure functions, use pattern matching, recursion, and Lists, define data types, and execute IO operations.

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Functional programming is a problem-solving paradigm that differs drastically from the imperative programming style of languages like C++ or Java. While functional programming principles have invaded most modern programming languages to some degree, there is no language that embraces this paradigm quite as Haskell does. As a result, Haskell code can reach a level of expressiveness, safety, and elegance that is difficult to achieve in other programming languages.

This course will introduce you to the core concepts of functional programming in Haskell. You will learn how to write pure functions using techniques such as pattern matching and recursion, and how to work with Haskell's powerful List data type. You will understand the basics of Haskell's powerful type system and define your own data types. Finally, you’ll learn how to perform IO operations the Haskell way. By the end of this course, you’ll have a new paradigm to add under your belt and you can start using functional programming in your own projects.

Learn Functional Programming in Haskell

In the last lesson, we learned how to perform `IO` operations in Haskell and separate them from pure code. We also saw how to combine `IO` operations in sequence by using the `>>` and `>>=` operators. But chaining many `IO` actions this way tends to look clumsy and unreadable, especially when many `>>=` operators and lambdas are involved. Luckily, Haskell offers a syntactic sugar variant for this called `do`-notation.

# Introducing `do`-notation.

As an example, let's take a simple program that asks the user to enter two numbers and add them, combining several `IO` actions.

```
addInput :: IO ()
addInput = putStrLn "What's your first number?" 
                 >> readLn 
                 >>= \first -> (
                   putStrLn "What's your second number?" 
                   >> readLn
                   >>= \second -> putStrLn ("The sum is " ++ (show (first + second)) ++ "!")
                 )
```
With the nested lambda expressions capturing the output of `readLn`, this becomes tricky to decipher at first glance. We could split it into several functions to improve readability, but a simpler solution is to use `do`-notation.

```
addInput :: IO ()
addInput = do
  putStrLn "What's your first number?" 
  first <- readLn
  putStrLn "What's your second number?" 
  second <- readLn
  putStrLn ("The sum is " ++ (show (first + second)) ++ ".")
```
To use `do`-notation, we write `do` to the right side of a function equation and then put each `IO` action we want to use on a single line. For `IO` actions that return an output that we want to use (like `getLine`), we can bind the output to a variable like in `first <- getLine`. The `String` yielded by `getLine :: IO String` will be bound to `first`, which can then be used in later `IO` actions.

Here is a terminal in which you can run the `addInput` IO action.

In this lesson, we study do-notation, a more convenient way to chain IO actions.

Introduction

First Steps with Haskell

Techniques for Writing Functions

Working with Lists

Creating Data Types

Input and Output

Conclusion

Appendix

Do Notation

Introducing `do`-notation.

Do Notation

Introducing do-notation.

Introducing `do`-notation.