Instantiation Errors

Hello,

I'm trying to reference my full adder module inside another module but seem to be getting lost somehow in the referencing. I keep getting the error Checker 'Full_Adder' not found. Instantiation 's' must be of a visible checker.

the code I have is:

always @(posedge clk)

begin

if (reset==1) // reset isn't pressed

begin

if (enable==1) //enable is active

begin

case ({ALU_OP})

2'h0 : D = A & B;

2'h1 : D = A | B;

2'h2 : Full_Adder f(A,B,D,C);

2'h3 : D = A * B;

endcase

end

end

end

endmodule

module Full_adder(a,b,answer,carry_out);

//inputs

input [2:0] a,b;

//outputs

output [2:0] answer;

output carry_out;

//wires

wire carry_out;

wire [2:0] carry;

genvar i;

generate

for(i=0;i<2;i=i+1)

begin: generate_N_bit_Adder

if(i==0)

half_adder f(a[0],b[0],answer[0],carry[0]);

else

full_adder f(a,b,carry[i-1],answer,carry);

end

assign carry_out = carry[2];

endgenerate

endmodule