Haskell tutorial for beginners

Prerequisites

• You can code on one of the imperative programming languages (C, C++, Python, Java, ...)
• You have installed Haskell Toolchain (ghc, ghci, cabal)

Content

1. Introduction
2. Basics
3. Lists

Introduction

What is Functional Programming?

• Pure functions - aka mathematical functions, they can not have side effects.
• Immutable data - values cannot be reassigned.
• Declarative - express the logic instead of control flow (for example, SQL is declarative).
• Easy to verify - you can mathematically prove the correctnes of an algorithm.

Declarative vs Imperative

Imperative
Describe an algorithm how to sum an array. You give an instructions to machiene.
Java

int sum (int[] arr) {
    int sum = 0;
    for (int i = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum;
}

Declarative
Define what it means to have a sum.
Haskell

sum [] = 0
sum (x:xs) = x + sum xs

Lazy vs Strict

Consider scenario: we have 3 functions: func1, func2, func3. Every functions needs 1 hour to run.

Haskell is lazily evaluated. When variables are defined, they are not evaluated. They evaluates only when they are used. In the code below, only x or y is evaluated, depending on z, but never both. It means that our code will run in 2 hours.
Haskell

evaluate arg =
    let x = func1 arg
        y = func2 arg
        z = func3 arg
    in
    if z then x else y

Strict evaluation
Variables are evaluated immediately. It means when we call func on an assignment, will hold the evaluated value of the func. In our scenario, our code will run in 3 hours. Even though, we will use only x or y as a return value.
Java

int evaluate(int arg) {
    int x = func1(arg);
    int y = func2(arg);
    int z = func3(arg);

    if (z) {
        return x;
    } else {
        return y;
    }
}