Haskell Practice Exercises: Essential Techniques for Mastering the Framework

Here’s a short practice exercise to help you solidify your understanding of Haskell:

Exercise: Fibonacci sequence

In this exercise, you’ll write a Haskell function to generate the Fibonacci sequence. The Fibonacci sequence is a series of numbers where each number is the sum of the two preceding numbers. The first two numbers in the sequence are 0 and 1, and the sequence continues indefinitely.

1. Define a Haskell function called fibonacci that takes an integer n and returns a list containing the first n numbers of the Fibonacci sequence.
2. Here’s an example of what the fibonacci function should return for different values of n:

cssCopy codefibonacci 0 => []
fibonacci 1 => [0]
fibonacci 2 => [0, 1]
fibonacci 5 => [0, 1, 1, 2, 3]

3. Once you’ve defined your fibonacci function, try using it to generate the first 20 numbers of the Fibonacci sequence.

Solution

Here’s a possible solution to the exercise:

haskellCopy codefibonacci :: Int -> [Int]
fibonacci 0 = []
fibonacci 1 = [0]
fibonacci n = take n fibs
  where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

In this solution, we use pattern matching to handle the cases where n is 0 or 1, and we use the take function to generate a list containing the first n numbers of the Fibonacci sequence.

The actual sequence is generated using the fibs list, which starts with the values 0 and 1, and then uses zipWith to generate the rest of the sequence. The zipWith function takes two lists and applies a function to each pair of elements to generate a new list. In this case, we’re using the + function to add the previous two elements of the fibs list together.

To generate the first 20 numbers of the Fibonacci sequence, you can simply call the fibonacci function with an argument of 20:

yamlCopy code*Main> fibonacci 20
[0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181]

This should generate a list containing the first 20 numbers of the Fibonacci sequence.