Question

I’m currently making a chess game for a school assignment and my pieces are defined

    data Piece = Piece { piecetype :: PieceType, color :: PieceColor }
    data PieceType = Pawn | Knight | Bishop | Rook | Queen | King
    data PieceColor = Black | White deriving Eq

now I have to print a piece as a single char (king = k, queen = q, knight = n, etc.) for black pieces the value is a capital of its white value (king = K, queen = Q, knight = N, etc.)
So I made three instances of show

    instance Show PieceColor where
        show Black = "B"
        show White = "W"

    instance Show PieceType where
        show Pawn = "P"
        show Knight = "N"
        show Bishop = "B"
        show Rook = "R"
        show Queen = "Q"
        show King = "K"

and the third one is the problem

    instance Show Piece where
        show (piecetype, color) = show piecetype 
                                  if show color == "W"
                                  then show piecetype
                                  else toUpper (show piecetype)

I receive the following error (also I tried a lot more than this but according to this link I seem pretty close Something somewhat similar

Couldn't match expected type `Piece' with actual type `(t0, t1)'

I appreciate any help
Kind regards, Me

Answer 1

Direct Answer:

instance Show Piece where
    show (Piece piecetype Black) = map toLower $ show piecetype
    show (Piece piecetype White) = show piecetype

Explanations:

• Piece is a datatype with constructor Piece (both type name and constructor are same for your case) and not a tuple. So you should pattern match on constructor and not on tuple.

• toUpper and toLower are present in Data.Char and work on Char while things like "B" are String ( String is type synonym for [Char]). So to make a String uppercase you can use map toUpper.

• Comparing string like show color == "B" is not very efficient instead you can use pattern matching on the constructor names.