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

# Parsing and printing

So far, we have used `getLine` and `putStrLn` to read and write lines as strings. But, we will need to read and work with input of other types, such as numbers. In that case, we need to convert them from strings after reading and convert them back to strings before writing.

As we have learned in the lesson on basic data types, we can use the `read` and `show` functions for such conversions. Here is a function that reads a number and doubles it.

```
ioDouble :: IO ()
ioDouble = getLine >>= \s -> putStrLn ((show . (*2) . read) s)
```
This is clumsy. Thankfully, there are two utility functions from the Prelude that make this easier:

- `readLn` combines `getLine` and `read`
- `print` combines `show` and `putStrLn`

Using these, our doubling program becomes easier to read.
```
ioDouble :: IO ()
ioDouble = readLn >>= \a -> print (a*2)
```

# Reading and writing files

Instead of interfacing with the terminal, we can also read and write files as strings. The `IO` actions both take a filename as the first argument.

```
readFile :: String -> IO String
writeFile :: String -> String -> IO ()
```

Here is a function which copies a file:
```
copy :: String -> String -> IO ()
copy source dest = readFile source >>= writeFile dest
``` 
Note that we are using a partial application of `writeFile` here. An equivalent, but more verbose version would be
```
copy :: String -> String -> IO ()
copy source dest = readFile source >>= \s -> writeFile dest s
``` 

Get introduced to a few more of the most common `IO` actions from the Prelude, and learn how to combine them with pure code.

Introduction

First Steps with Haskell

Techniques for Writing Functions

Working with Lists

Creating Data Types

Input and Output

Conclusion

Appendix

More IO Functions

Parsing and printing