# Haskell Community

# Haskell.Community

At Haskell.Community, our mission is to provide a comprehensive resource for individuals interested in the Haskell programming language. We strive to create a welcoming and inclusive community where users can learn, share, and collaborate on all aspects of Haskell development. Our goal is to promote the use of Haskell as a powerful and elegant tool for solving complex problems, and to foster a culture of innovation and creativity within the Haskell community.

### Video Introduction Course Tutorial

# /r/Haskell Yearly

Introduction

Haskell is a functional programming language that has been around since the late 1980s. It is a statically typed language that is known for its strong type system, lazy evaluation, and purity. Haskell is a popular language for academic research and is used in industries such as finance, telecommunications, and gaming. This cheat sheet is designed to provide a quick reference guide for anyone who is getting started with Haskell.

Getting Started

To get started with Haskell, you will need to install the Haskell Platform. The Haskell Platform is a collection of tools and libraries that are used for Haskell development. It includes the Glasgow Haskell Compiler (GHC), which is the most widely used Haskell compiler, as well as other tools such as Cabal, which is a package manager for Haskell.

Once you have installed the Haskell Platform, you can start writing Haskell code. Haskell code is typically written in a text editor and saved with a .hs file extension. To compile and run Haskell code, you can use the GHC compiler. To compile a Haskell file, you can use the following command:

```
ghc -o <output-file> <input-file>
```

This will compile the Haskell file and create an executable file with the specified output file name. To run the executable file, you can use the following command:

```
./<output-file>
```

Basic Syntax

Haskell code is made up of expressions, which are evaluated to produce a result. Expressions can be simple values, such as numbers or strings, or they can be more complex expressions that combine values using operators and functions.

Here are some examples of basic Haskell expressions:

```
-- Numbers
1
2.5
-3
-- Strings
"Hello, world!"
"123"
-- Boolean values
True
False
-- Lists
[1, 2, 3]
["hello", "world"]
-- Tuples
(1, "hello")
(2.5, False, "world")
```

Variables

In Haskell, variables are defined using the `let`

keyword. Here is an example of defining a variable:

```
let x = 5
```

This defines a variable `x`

with the value of `5`

. Once a variable is defined, it cannot be changed. This is because Haskell is a pure functional language, which means that functions cannot have side effects and variables cannot be mutated.

Functions

Functions are a fundamental part of Haskell programming. Functions are defined using the `function-name argument1 argument2 ... = expression`

syntax. Here is an example of a simple function:

```
double x = x * 2
```

This defines a function called `double`

that takes a single argument `x`

and returns the result of multiplying `x`

by `2`

. Functions can be called by passing arguments to them. Here is an example of calling the `double`

function:

```
double 5
```

This will return the value `10`

.

Higher-Order Functions

Haskell is a functional programming language, which means that functions are first-class citizens. This means that functions can be passed as arguments to other functions and can be returned as values from functions. Functions that take other functions as arguments or return functions as values are called higher-order functions.

Here is an example of a higher-order function:

```
applyTwice f x = f (f x)
```

This defines a function called `applyTwice`

that takes two arguments: a function `f`

and a value `x`

. The function `applyTwice`

applies the function `f`

twice to the value `x`

and returns the result. Here is an example of calling the `applyTwice`

function:

```
applyTwice double 5
```

This will return the value `20`

.

Lists

Lists are a fundamental data structure in Haskell. Lists are defined using square brackets and can contain any number of elements of any type. Here is an example of a list:

```
[1, 2, 3, 4, 5]
```

Lists can be manipulated using a variety of functions. Here are some examples of list functions:

```
-- Get the length of a list
length [1, 2, 3, 4, 5]
-- Get the first element of a list
head [1, 2, 3, 4, 5]
-- Get the last element of a list
last [1, 2, 3, 4, 5]
-- Get all but the first element of a list
tail [1, 2, 3, 4, 5]
-- Get all but the last element of a list
init [1, 2, 3, 4, 5]
-- Check if a list is empty
null []
-- Reverse a list
reverse [1, 2, 3, 4, 5]
-- Concatenate two lists
[1, 2, 3] ++ [4, 5]
-- Get the element at a specific index in a list
[1, 2, 3, 4, 5] !! 2
-- Get all elements in a list that satisfy a predicate
filter (\x -> x > 2) [1, 2, 3, 4, 5]
-- Apply a function to each element in a list
map (\x -> x * 2) [1, 2, 3, 4, 5]
-- Fold a list from the left using a binary operator
foldl (\acc x -> acc + x) 0 [1, 2, 3, 4, 5]
-- Fold a list from the right using a binary operator
foldr (\x acc -> x + acc) 0 [1, 2, 3, 4, 5]
```

Tuples

Tuples are another data structure in Haskell. Tuples are similar to lists, but they can contain a fixed number of elements of different types. Tuples are defined using parentheses and commas to separate the elements. Here is an example of a tuple:

```
(1, "hello", True)
```

Tuples can be manipulated using a variety of functions. Here are some examples of tuple functions:

```
-- Get the first element of a tuple
fst (1, "hello", True)
-- Get the second element of a tuple
snd (1, "hello", True)
-- Combine two tuples into a single tuple
(1, "hello") `zip` (2, "world")
-- Apply a function to each element in a tuple
fmap (\x -> x * 2) (1, 2, 3)
-- Convert a list of tuples into a tuple of lists
unzip [(1, "hello"), (2, "world")]
```

Type Classes

Haskell has a powerful type system that is based on type classes. Type classes are a way of defining a set of functions that can be used with a particular type. For example, the `Num`

type class defines a set of functions that can be used with numeric types such as `Int`

and `Double`

.

Here are some examples of type classes:

```
-- The Num type class defines a set of functions that can be used with numeric types
class Num a where
(+) :: a -> a -> a
(-) :: a -> a -> a
(*) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
-- The Eq type class defines a set of functions that can be used for equality testing
class Eq a where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
-- The Ord type class defines a set of functions that can be used for ordering
class Ord a where
compare :: a -> a -> Ordering
(<) :: a -> a -> Bool
(<=) :: a -> a -> Bool
(>) :: a -> a -> Bool
(>=) :: a -> a -> Bool
max :: a -> a -> a
min :: a -> a -> a
```

Type Signatures

In Haskell, functions have type signatures that specify the types of their arguments and return values. Type signatures are used by the compiler to check that functions are used correctly.

Here is an example of a function with a type signature:

```
double :: Int -> Int
double x = x * 2
```

This defines a function called `double`

that takes an `Int`

argument and returns an `Int`

value. Type signatures are optional, but they are recommended because they make code easier to read and understand.

Conclusion

Haskell is a powerful programming language that is well-suited for functional programming. This cheat sheet provides a quick reference guide for anyone who is getting started with Haskell. It covers basic syntax, variables, functions, higher-order functions, lists, tuples, type classes, and type signatures. With this cheat sheet, you should be able to start writing Haskell code and exploring the many features of this fascinating language.

### Common Terms, Definitions and Jargon

1. Haskell: A purely functional programming language that is used for developing complex software applications.2. Functional programming: A programming paradigm that emphasizes the use of functions and avoids changing state and mutable data.

3. Pure function: A function that always returns the same output for a given input and has no side effects.

4. Side effect: A change in the state of a program that is not reflected in the return value of a function.

5. Immutable data: Data that cannot be changed once it has been created.

6. Higher-order function: A function that takes one or more functions as arguments or returns a function as its result.

7. Lambda function: An anonymous function that can be used as a value in expressions.

8. Type inference: The process of deducing the type of a value or expression from its context.

9. Type signature: A declaration that specifies the type of a function or value.

10. Polymorphism: The ability of a function or value to work with values of different types.

11. Type class: A collection of types that share a common set of operations.

12. Monoid: A type class that defines an associative binary operation and an identity element.

13. Functor: A type class that defines a mapping operation over a container.

14. Applicative: A type class that defines a way to apply a function to values inside a container.

15. Monad: A type class that defines a way to sequence computations and handle side effects.

16. Lazy evaluation: A technique where expressions are not evaluated until their values are needed.

17. Strict evaluation: A technique where expressions are evaluated immediately.

18. Recursion: A technique where a function calls itself to solve a problem.

19. Tail recursion: A form of recursion where the recursive call is the last operation in the function.

20. Memoization: A technique where the results of expensive function calls are cached and reused.

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