Pangram Checker in Haskell

Question

I was wondering how to check whether a string is a pangram, as a beginner's exercise. I have two solutions, but I don't understand them at all.

Version 1

import Data.Char (toLower)

isPangram :: String -> Bool
isPangram str = all (`elem` map toLower str) ['a'..'z']

Version 2

import Data.Char (toLower)

isPangram :: String -> Bool
isPangram str = allPresent (map toLower str) ['a'..'z']

allPresent :: String -> String -> Bool
allPresent _ [] = True
allPresent [] _ = False
allPresent (x:xs) ys
    | x `elem` ys = allPresent xs ys
    | otherwise = False

Not sure if they're correct, but any help to simplify or modify this would be appreciated!

Well, you will certainly confuse yourself if you try to understand how the second snippet is a pangram checker, because it is not one. — Daniel Wagner
– Daniel Wagner, Commented Mar 22, 2024 at 16:05

willeM_ Van Onsem · Accepted Answer · 2024-03-22 16:54:26Z

1

Your second version is not. That should be:

isPangram :: String -> Bool
isPangram str = allPresent ['a' .. 'z'] (map toLower str)

allPresent :: String -> String -> Bool
allPresent [] _ = True
allPresent _ [] = False
allPresent (x : xs) ys
  | x `elem` ys = allPresent xs ys
  | otherwise = False

Indeed, for a pangram you check if all characters belong to the string. But that is not symmetrical: checking if all elements of X belong to Y is not equivalent to checking if all elements of Y belong to X. We here thus use the first list as the collection of items where we check if each item x of that collection, is an element of the second collection ys.

We thus call allPresent ['a' .. 'z'] (map toLower str) so the first list contains all lowercase characters, the second the lowercase version of the input str. In case the first list is empty, we are done, so we can return True. In case it is not, and the second list is empty, we know it will be False. If both lists are not empty, we check if the first item x of the list is a member of ys, and if so recurse with the rest of the elements xs; otherwise we can return False.

answered Mar 22, 2024 at 16:54

willeM_ Van Onsem

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4 Comments

just coding Over a year ago

Is there any way to implement this without using an external module like Data.char? Also accept a list of characters, and not string? like this isPangram :: [Char] -> Bool

willeM_ Van Onsem Over a year ago

String and [Char] are the same, so it works already with isPangram :: [Char] -> Bool, Data.Char is not an external module, it is part of the base, and thus always "'ships" with the basic library. You could implement toLower yoursel with toLower 'A' = 'a'; toLower 'a' = 'a'; toLower 'B' = 'b', etc.

just coding Over a year ago

Could you please add that to the answer above? It's hard for me to understand so I would appreciate any help

just coding Over a year ago

Nvm, I figured it out so! Thanks anyway!

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