Null Pointers, Asserts and Warnings

Null pointers

So you've got a survey on your website which asks your readers for their names and ages. Only problem is that for some reason a few of your readers don't want to give you their age - they stubbornly refuse to fill that field in. What's a poor database administrator to do?

Assume that the age is represented by an int, there are two possible ways to solve this problem. The most common one (and the most wrong one) is to assume some sort of "special" value for the age which means that the age information wasn't collected. So if, say, age = -1 then the data wasn't collected, otherwise the data was collected (even if it's not valid!). This method kind of works until you start, for example, calculating the mean age of visitors to your website. Since you forgot to take into account your special value, you conclude that the mean age of visitors is 7½ years old, and you employ web designers to remove all the long words and use primary colours everywhere.

The other, correct method is to store the age in a field which has type "int or null". Here's a SQL table for storing ages:

create table users
  userid serial,
  name text not null,
  age int             -- may be null

If the age data isn't collected, then it goes into the database as a special SQL NULL value. SQL ignores this automatically when you ask it to compute averages and so on.

Programming languages also support nulls, although they may be easier to use in some than in others. In Perl, any scalar (ie. number or string) can be undef (Perl's way of saying null). This is a cause of many warnings which are often ignored by inexperienced programmers, even though they probably represent serious errors. In Java, any reference to an object can also be null, so it might make sense in Java to store the age as an Integer and allow references to the age to be null. In C pointers can, of course, be null, but if you wanted a simple integer to be null, you'd have to first box it up into an object allocated by malloc on the heap.

OCaml has an elegant solution to the problem of nulls, using a simple polymorphic variant type defined (in Pervasives) as:

type 'a option = None | Some of 'a

A "null pointer" is written None. The type of age in our example above (an int which can be null) is int option [remember: backwards like int list and int binary_tree].

# Some 3;;
- : int option = Some 3

What about a list of optional ints?

# [ None; Some 3; Some 6; None ];;
- : int option list = [None; Some 3; Some 6; None]

And what about an optional list of ints?

# Some [1; 2; 3];;
- : int list option = Some [1; 2; 3]

Assert, warnings, fatal errors, and printing to stderr

One great feature of Perl is the rich set of commands for debugging programs and handling unexpected errors, including the ability to print stack traces, throw and catch exceptions and the like. OCaml doesn't have quite such a rich set of debugging commands - better than Java, about the same as C, not nearly as good as Perl. (We'll talk about exceptions in more detail later on.)

First of all, assert takes an expression as an argument and throws an exception. Assuming that you don't catch this exception (it's probably unwise to catch this exception, particularly for beginners), this results in the program stopping and printing out the source file and line number where the error occurred. An example:

# assert (Sys.os_type = "Win32");;
Exception: Assert_failure ("", 0, 30).

(Running this on Win32, of course, won't throw an error).

You can also just call assert false to stop your program if things just aren't going well, but you're probably better to use ...

failwith "error message" throws a Failure exception, which again assuming you don't try to catch it, will stop the program with the given error message. failwith is often used during pattern matching, like this real example:

  match Sys.os_type with
    "Unix" | "Cygwin" ->   (* code omitted *)
  | "Win32" ->             (* code omitted *)
  | "MacOS" ->             (* code omitted *)
  | _ -> failwith "this system is not supported"

Note a couple of extra pattern matching features in this example too. A so-called "range pattern" is used to match either "Unix" or "Cygwin", and the special _ pattern which matches "anything else".

If you want to debug your program, but, like me, you have an aversion to debuggers which aren't gdb, then you'll probably want to print out a warning some way through your function. Here's an example (note the code highlighted in red):

open Graphics;;

open_graph " 640x480";;
for i = 12 downto 1 do
  let radius = i * 20 in
  prerr_endline ("radius is " ^ (string_of_int radius));
  set_color (if (i mod 2) = 0 then red else yellow);
  fill_circle 320 240 radius
read_line ();;

If you prefer C-style printf's then try using OCaml's Printf module instead:

open Graphics;;
open Printf;;

open_graph " 640x480";;
for i = 12 downto 1 do
  let radius = i * 20 in
  eprintf "radius is %d\n" radius;
  set_color (if (i mod 2) = 0 then red else yellow);
  fill_circle 320 240 radius
read_line ();;

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