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

# Currying functions with more arguments

The technique of currying is not limited to functions with two arguments. If we have a function with three arguments like this:


isTriangleTuple :: (Double, Double, Double) -> Bool
isTriangleTuple (alpha, beta, gamma) = alpha + beta + gamma == 180

main = print (isTriangleTuple (90, 45, 45))

we also can turn it into its curried form:


isTriangle :: Double -> Double -> Double -> Bool
isTriangle alpha beta gamma = alpha + beta + gamma == 180

main = print (isTriangle 90 50 50)

and we can use a partial application with one or two arguments.
```
mustBeSixty = isTriangle 100 20
sumToNinety = isTriangle 90
```

# The functions `curry` and `uncurry` 
Now, let's briefly look at the functions `curry` and `uncurry` from the Haskell Prelude. They are higher order functions that can be used to transform functions between their curried and uncurried forms.

```
*Divides> :t curry dividesTuple
Int -> Int -> Bool
*Divides> :t uncurry divides
(Int, Int) -> Bool
```
This allows us to do partial applications even for functions that were not defined as curried.
```
*Divides> divisibleByFour = (curry dividesTuple) 4
*Divides> divisibleByFour 8
True
```
Again, parentheses here are optional but added for clarity. The functions `curry` and `uncurry` only work for functions with two arguments.

# Exercise: Currying

As an exercise, consider the function below
```
inRange :: (Int, Int, Int) -> Bool
inRange (lower, upper, query) = lower <= query && query <= upper
```

1. Write the curried form of `inRange`.
2. Using partial application, also define a function `betweenTenAndTwenty` that takes one integer argument and checks whether it is between `10` and `20`, inclusive.

Learn how to transform functions into their curried or uncurried forms.

Introduction

First Steps with Haskell

Techniques for Writing Functions

Working with Lists

Creating Data Types

Input and Output

Conclusion

Appendix

Currying and Uncurrying