is there a way to define if a word has only the "a" vowel?

Question

im trying to make a function on haskell, I know that the function is define like:

justWithA : [Char] -> Bool
justWithA [] = True
justWithA (x:xs) | (x == "a") = x + justWithA xs                 
                 | else (x == "e") = False
                 | else (x == "i") = False
                 | else (x == "o") = False
                 | else (x == "u") = False

I think there is a better way to define this function, just don't know the correct way to do it...

Answer 1

There are some problems with the current implementation:

• a guard does not use an else keyword;
• x is a Char, so you compare with x == 'a', not x == "a";
• x + justWithA xs makes no sense since x is a Char and you can not add a Char and Bool together; and
• your function will error if the string contains consonant, since these are not covered by any guard.

You can fix the function with:

justWithA : String -> Bool
justWithA [] = True
justWithA (x:xs)
    | x == 'a' = justWithA xs
    | x == 'e' = False
    | x == 'i' = False
    | x == 'o' = False
    | x == 'u' = False
    | otherwise = justWithA xs

we here only look for vowels and if there is a vowel that is e, i, o, or u, then we return False.

We can simplify this to:

justWithA :: String -> Bool
justWithA = all (`notElem` "eiou")

or if you want to take uppercase into account as well:

justWithA :: String -> Bool
justWithA = all (`notElem` "eiouEIOU")