Convert Strings into my own data type with Haskell-CodePudding

I'm working on a Haskell function that is going to to convert a string to a self-defined data type : Position, which should only consist a char of (A-H) and an Int(1-4).(i.e A1, B3, H4)

Here is the function's usage: toPosition gives Just the Position named by the string, or Nothing if the string is not a valid Position name.

Here is my try:

I defined the data type as:

data Position = Pos Char Int

My attempt on toPosition:

import Data.Char
toPosition :: String -> Maybe Position
toPosition string = case string of
  first : second : rest ->
    if isValidChar first
      then if isValidInt second
        then if null rest
          then Just (Pos first (ord second))
          else Nothing -- more than two characters
        else Nothing -- invalid second character
      else Nothing -- invalid first character

isValidChar :: Char -> Bool
isValidChar x
    |x == 'A' = True
    |x == 'B' = True
    |x == 'C' = True
    |x == 'D' = True
    |x == 'E' = True
    |x == 'F' = True
    |x == 'G' = True
    |x == 'H' = True
    |otherwise = False

isValidInt :: Char -> Bool
isValidInt x
    |x == '1' = True
    |x == '2' = True
    |x == '3' = True
    |x == '4' = True
    |otherwise = False

I tried to run it with this in the terminal:

toPosition "H2"

GHCi returning ERROR:

No instance for (Show Position) arising from a use of ‘print’
In a stmt of an interactive GHCi command: print it

Does anyone have ideas about this? Any help would be appreciated!

CodePudding user response：

OK, I've basically got it done.

Just added deriving (Show,Eq) and a anythingElse statement will solve the problem.

Thanks for everyone that helped me!! Love from Italy

CodePudding user response：

The reason this raises an error is because it can not print the Position, since you did not make it an instance of Show. You thus can implement this as:

data Position = Pos Char Int deriving Show

But your functions are also quite complicated. You can simplify isValidChar and isValidInt with:

isValidChar :: Char -> Bool
isValidChar x = 'A' <= x && x <= 'H'

isValidInt :: Char -> Bool
isValidInt x = '1' <= x && x <= '4'

For the toPosition function, you can use pattern matching on a list, since a String is a list of Chars:

import Data.Char(digitToInt)

toPosition :: String -> Maybe Position
toPosition [c, n]
    | isValidChar c && isValidInt n = Just (Position c (digitToInt n))
toPosition _ = Nothing