How to fix: 'Argument of Type Conversion Must Be Single Expression' in Ada 95

Question

I am writing a simple program in Ada95 to check to see if a chess board layout input is valid or not. I am quite new to Ada and I understand I have a lot to learn. The error messages that I keep getting are:

35:31: argument of type conversion must be single expression

47:07: argument of type conversion must be single expression

47:22: illegal operand for array conversion

53:10: argument of type conversion must be single expression

58:10: argument of type conversion must be single expression

61:13: argument of type conversion must be single expression

64:13: argument of type conversion must be single expression

66:13: argument of type conversion must be single expression

68:13: argument of type conversion must be single expression

76:15: invalid use of subtype mark in expression or call

I have included my full code below so that if there is anything that could be causing an issue in a declaration, it is available for reference.

-- RULES:
-- Only black squares may have pieces on them
-- Black squares start at the bottom left hand corner
-- Total checker count for a color cannot exceed 12
-- A color may not have more than 7 kings

with Ada.Text_IO, Ada.Integer_Text_IO;
use Ada.Text_IO, Ada.Integer_Text_IO;

procedure Checker_Checker is
  type Checkers_Piece is ("b", "BK", "RK", "BC", "RC");
  type Checkers_Board is array (1..8, 1..8) of Checkers_Piece;
  Checkers_String: String(1..64);
  invalid, 
    BK_Count, -- black king checker
    RK_Count, -- red king checker
    RC_Count, -- red checker
    BC_Count, -- black checker
    b_Count, -- blank space
    Char_Count      : Integer := 0;
  
  procedure Print_Board (Object: in Checkers_Board) is
    Print_Divider: String(1..17);
    Print_Squares: String(1..17);
  begin
    Print_Divider := "-----------------";
    Print_Squares := "| | | | | | | | |";
    for X in 1..8 loop
      Put_Line(Print_Divider);
      for Y in 1..8 loop
        Print_Squares(Y+1) := Checkers_Board(X, Y);
      end loop;
      Put_Line(Print_Squares);
    end loop;
    Put_Line(Print_Divider);
  end Print_Board;
  
begin
  Get(Checkers_String);
  for I in 1..8 loop
    for J in 1..8 loop
      Char_Count := Char_Count + 1;
      Checkers_Board(I, J) := Checkers_String(Char_Count);
    end loop;
  end loop;
  
  for Y in 1..8 loop
    for X in 1..8 loop
      if Checkers_Board(Y, X) = 'b' then
        Put_Line("There is a piece on a white space.");
        invalid := invalid + 1;
      end if;

      if Checkers_Board(Y, X) = "BK" then
        BK_Count := BK_Count + 1;
        BC_Count := BC_Count + 1;
      elsif Checkers_Board(Y, X) = "RK" then
        RK_Count := RK_Count + 1;
        RC_Count := RC_Count + 1;
      elsif Checkers_Board(Y, X) = "RC" then
        RC_Count := RC_Count + 1;
      elsif Checkers_Board(Y, X) = "BC" then
        BC_Count := BC_Count + 1;
      elsif Checkers_Board(Y, X) = "b" then
        b_Count := b_Count + 1;
      else
        Put_Line("There is an  unidentified character on the board.");
      end if;
    end loop;
  end loop;
  
  Print_Board;
  
  if RK_Count > 7 then
    Put_Line("There are more than 7 Red Kings on the board.");
  end if;
  
  if BK_Count > 7 then
    Put_Line("There are more than 7 Black Kings on the board.");
  end if;
  
  if RC_Count > 12 then
    Put_Line("There are more than 12 Red Checkers on the board.");
  end if;
  
  if BC_Count > 12 then
    Put_Line("There are more than 12 Black Checkers on the board.");
  end if;
  
  if b_Count < 32 then
    Put_Line("There are too many checkers on the board.");
  end if;
  
  if invalid = 0 then
    Put_Line("This is a valid checker board.");
  else
    Put_Line("This is an invalid checker board.");
  end if;
end Checker_Checker;

I know my code is a mess.. please don't be too critical of it.. I am trying my best at learning Ada. Thank you for your help.

Answer 1

For a start,

type Checkers_Piece is ("b", "BK", "RK", "BC", "RC");

is trying to declare an enumeration; but enumeration literals are identifiers, not strings. So that should be

   type Checkers_Piece is (b, BK, RK, BC, RC);

Then, in

    Print_Squares := "| | | | | | | | |";
    for X in 1..8 loop
      Put_Line(Print_Divider);
      for Y in 1..8 loop
        Print_Squares(Y+1) := Checkers_Board(X, Y);
      end loop;
      Put_Line(Print_Squares);
    end loop;

you were clearly expecting Checkers_Board(X, Y) to be a single Character, but in your declaration you intended it to be a sort of String, with up to 2 Characters.

You have to decide what representation you want that lets you distinguish a red king from a black king in a single character. Or, better, accept that you need 2 characters to represent your pieces. Or perhaps you could use single Characters with the convention that lower-case values are uncrowned, upper-case are crowned, space means unoccupied.

The reason for your 'argument of type conversion' errors is that, in lines like

   Checkers_Board(I, J) := Checkers_String(Char_Count);

Checkers_Board is a type; you need an object of that type here, not the type itself. The syntax you’re using is for a type conversion (ARM 4.6).