problem.barrett

{ Sieve of Eratosthenes without assignment.
* Note: Assignment is used only in Cons function and to return values from functions
* since barrett returns values by assigning to the special variable 'Result'
}

function Cons(A, B);
    var
        ConsCell[0..1];
    begin
        ConsCell[0] := A;
        ConsCell[1] := @B;
        Result := ConsCell;
    end;

function ConsStream(A, B);
    begin
        Result := Cons(A, @B);
    end;

function StreamCar(Stream);
    begin
        Result := Stream[0];
    end;

function StreamCdr(Stream);
    begin
        Result := Stream[1];
    end;

function DisplayStream(S, N);
    begin
        if N > 0 then
        begin
            WriteLn(StreamCar(S));
            DisplayStream(StreamCdr(S), N-1);
        end;
    end;

function StreamFilter(Pred, Stream);
    begin
        if Pred(StreamCar(Stream)) then
        begin
            Result := ConsStream(StreamCar(Stream),
                lambda;
                begin
                    Result := StreamFilter(@Pred, StreamCdr(Stream));
                end;
                );
        end
        else
        begin
            Result := StreamFilter(@Pred, StreamCdr(Stream));
        end;
    end;

function IntegersStartingFrom(N);
    var
        ConsCell[0..1];
    begin
        Result := ConsStream(N,
            lambda;
            begin
                Result := IntegersStartingFrom(N+1);
            end;
            );
    end;

function IsNotDivisible(X, Y);
    begin
        Result := (X % Y) <> 0;
    end;

function Sieve(S);
    begin
        Result := ConsStream(StreamCar(S),
            lambda;
            begin
                Result := Sieve(StreamFilter(
                    lambda(x);
                    begin
                        Result := IsNotDivisible(x, StreamCar(S));
                    end;
                    , StreamCdr(S)));
            end;
            );
    end;

begin
    DisplayStream(Sieve(IntegersStartingFrom(2)), 12);
end;