Could you switch the argument name as well? It can be confusing.

next xs item = error "Not implemented"

to:

next item xs = error "Not implemented"

+1 for stop.

Actually, only one is given in one of the test cases when calling Plugboard() without any arguments for testing empty construction. Honestly, I think it should be Plugboard('') for empty pairing.

:(

Radii is the plural form of radius (thanks to the irregularity of English).

The test case does not test for input validation.

Write a function called sortGiftCode (sort_gift_code in Ruby) that accepts a string containing up to 26 unique alphabetical characters, and returns a string containing the same characters in alphabetical order.

Who likes keywords? Nobody likes keywords, so why use them?

Haskell's function arguments are switched.

It is currenty:

next :: Eq a => [a] -> a -> Maybe a
next xs item = error "Not implemented"

while it should be:

next :: Eq a => a -> [a] -> Maybe a
next item xs = error "Not implemented"

In Haskell, the type checking freaks out because the solution has type of Int -> String, but it gets Integer as input for test cases.

So, the type of solution should be either:

solution :: Integer -> String

or more generally:

solution :: Integral a => a -> String

Would you kindly put colons at the end of function definition in Python?

The Python one seems incomplete.

Test cases have error, and the solution test cases only check once.

The submit test has test case for empty list, but the test cases doesn't.

I think the wording is a bit vague. You should say, "you are not allowed to use Data.List or Data.Strings.Utils modules." (or something like that). I was puzzled because when you said join functions, I thought it meant functions starting with join which does not exist.

Typo: (>>=) :: m a -> (a -> m a) -> m b

should be: (>>=) :: m a -> (a -> m b) -> m b