How to limit a Int field to a range of values?

Question

In my Data

data User = User { uRank :: Int, uProgress :: Int }

I want to limit uRank to a list of values [-1, 1, 3], for example.

How do I do this?

Answer 1

Defining a small sum type is the best answer for this specific question. You can also use newtype with smart constructors to achieve this effect.

newtype Rank = UnsafeMkRank { unRank :: Int }

mkRank :: Int -> Maybe Rank
mkRank i 
    | i `elem` [-1, 1, 3] = Just (UnsafeMkRank i)
    | otherwise = Nothing

Now, provided you only use the safe mkRank constructor, you can assume that your Rank values have the Int values you want.