Pangram Problem - Haskell [Problem Solved]

Background

This problem comes courtesy of the great folks over at Exercism. I highly recommend jumping on their Haskell track to apply your knowledge of Haskell.

The Problem

Your task is to figure out if a sentence is a pangram.

A pangram is a sentence using every letter of the alphabet at least once. It is case insensitive, so it doesn’t matter if a letter is lower-case (e.g. k) or upper-case (e.g. K).

For this exercise, a sentence is a pangram if it contains each of the 26 letters in the English alphabet.

Knowledge You Need to Solve This Problem

Basic Functions and How to Write Them
Elem Function
Higher-order Functions
Maps & Filters
Pattern Matching

The Solution

To solve this problem in Haskell, you can create a function that checks if a given sentence contains every letter of the alphabet at least once. Here’s how you could do it:

Define the alphabet as a list of characters.
For each letter in the alphabet, check if it exists in the sentence using elem.
Ensure all letters are present to confirm it’s a pangram.

Here’s the Haskell code implementing this approach:

import Data.Char (toLower, isAlpha)

isPangram :: String -> Bool
isPangram sentence =
    let lowercaseSentence = map toLower sentence
        alphabet = ['a'..'z']
    in all (`elem` lowercaseSentence) alphabet

Explanation:

map toLower sentence converts the sentence to lowercase, making the check case-insensitive.
alphabet = ['a'..'z'] defines the list of all lowercase alphabet letters.
all (elem lowercaseSentence) alphabet checks if each letter in alphabet exists in lowercaseSentence by using elem for each letter.

Usage Example:

main :: IO ()
main = do
    print $ isPangram "The quick brown fox jumps over the lazy dog" -- Should return True
    print $ isPangram "Hello World"                                 -- Should return False

This solution will return True if the sentence is a pangram and False otherwise.

💡 Helpful References

Exercism - Pangram
https://exercism.org/tracks/haskell/exercises/pangram/

Pangram Problem – Haskell [Problem Solved]