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EDA Playground lets you type in and run HDL code (using a selection of free and commercial simulators and synthesizers).
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testbench.sv

 
module RCAS_TB;
  wire [3:0] S, Cout;
  reg [3:0] A, B;
  reg ctrl;
  
  ripple_carry_adder_subtractor rcas(A, B, ctrl, S, Cout);
  
  initial begin
    $monitor("CTRL=%b: A = %b, B = %b --> S = %b, Cout[3] = %b", ctrl, A, B, S, Cout[3]);
    ctrl = 0;
    A = 1; B = 0;
    #3 A = 2; B = 4;
    #3 A = 4'hb; B = 4'h6;
    #3 A = 5; B = 3;
    ctrl = 1;
    A = 1; B = 0;
    #3 A = 2; B = 4;
    #3 A = 4'hb; B = 4'h6;
    #3 A = 5; B = 3;
    #3 $finish;
  end
  
  initial begin
    $dumpfile("waves.vcd");
    $dumpvars;
  end
endmodule

design.sv

 
module full_adder(
  input a, b, cin,
  output sum, cout
);
  
  assign {sum, cout} = {a^b^cin, ((a & b) | (b & cin) | (a & cin))};
  //or
  //assign sum = a^b^cin;
  //assign cout = (a & b) | (b & cin) | (a & cin);
endmodule
​
module ripple_carry_adder_subtractor #(parameter SIZE = 4) (
  input [SIZE-1:0] A, B, 
  input CTRL,
  output [SIZE-1:0] S, Cout);
  bit [SIZE-1:0] Bc;
  genvar g;
  
  assign Bc[0] = B[0] ^ CTRL;
    
  full_adder fa0(A[0], Bc[0], CTRL, S[0], Cout[0]);
  generate  // This will instantial full_adder SIZE-1 times
    for(g = 1; g<SIZE; g++) begin
      assign Bc[g] = B[g] ^ CTRL;
      full_adder fa(A[g], Bc[g], Cout[g-1], S[g], Cout[g]);
    end
  endgenerate
endmodule

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