% Driver and function to compute factorials
% from the PCLU distribution.
 start_up = proc ()
    pi: stream := stream$primary_input()
    po: stream := stream$primary_output()
    while true do
        stream$puts(po, "Enter an integer (or RETURN to exit): ")
        s: string := stream$getl(pi)
        if string$empty(s) then break end
        n: int := int$parse(s)
           except when bad_format: 
                       stream$putl(po, "Illegal integer")
                  end
        stream$putl(po, int$unparse(n) || "! = " || int$unparse(factorial(n)))
           except when negative: 
                       stream$putl(po, "Integer must be positive")
                  when overflow:
                       stream$putl(po, "Overflow")
                  end
        end
    end start_up
 
factorial = proc (n: int) returns (int) signals (negative, overflow)
    if n < 0 then signal negative end
    if n = 0 then return(1) end
    return(n*factorial(n-1))
       resignal overflow
    end factorial