Null Pointers, Asserts and Warnings
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
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)
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
type 'a option = None | Some of 'a
A "null pointer" is written
None. The type of age in our example above
int which can be null) is
int option (remember: backwards like
int list and
# 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 ("", 1, 0).
(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
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
"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:
open Graphics let () = 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 done; read_line ()
If you prefer C-style
printf's then try using OCaml's
open Graphics open Printf let () = 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 done; read_line ()