"defensive programming" (Was: Re: How nice should I be on exit?)


Joe Armstrong <>?
Wed Mar 5 12:25:08 CET 2003
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On 5 Mar 2003, Luke Gorrie wrote:


... cut ...


> Hope that clears things up for someone else who learned the other
> definition than the Erlang guys :-)


Yes :-)


I have two "thumb rules"


Check inputs where they are "untrusted"?


- at a human interface
- a foreign language program


Or when you want a better error diagnostic that the default one -
in this case just exit with the better diagnostic.


For example if I'm parsing an integer I'd write


I = list_to_integer(L)


or


case (catch list_to_integer(L)) of
? ?{'EXIT', _} ->
exit(["Most honored user I regrettably have to inform you
? ? ?that your input on line", Ln, "was not an integer
? ? ?in fact it was ",L, "which IMHO is wrong
? ? ?have a nice day Mr. C. Computer"]);
? ?I ->I
end.


The latter is "an industrial quality" error message :-)


Note (important) the semantics of both are to raise an exception in the
event of an error.?

Aside: I once saw code like this:


x(a) -> 1;
x(b) -> 2;
x(X) -> io:format("expecting a or b").
%% what do I do now


The programmer had actually added a comment (What do I do now) -
of course they had done the wrong thing.


The program:


x(a) -> 1;
x(b) -> 2.


Is correct.


Evaluating x(c) generates an exception as required.


In their modified program x(c) evaluates to the atom 'ok' (i.e. the return
value of io:format) - which is incorrect.


If they had wanted a better diagnostic they should have written:


x(a) -> 1;
x(b) -> 2;
x(X) -> exit({x,expects,argument,'a or b'}).


If you do *nothing* to your code you get a good diagnostic anyway:


If x in in the module m and you call this in the shell
you'd get:


(catch m:x(c)).
{'EXIT',{function_clause,[{m,x,[c]},
? ? ? ? ? ? ? ? ? ? ? ? ? {erl_eval,expr,3},
? ? ? ? ? ? ? ? ? ? ? ? ? {erl_eval,exprs,4},
? ? ? ? ? ? ? ? ? ? ? ? ? {shell,eval_loop,2}]}}


function_clause means you couldn't match a function head.


[{m,x,[c]}, ...


means you were calling function x with argument c


So in this case NOT programming the error case ?results in


1) shorted code
2) clearer code
3) no chance of accidentally violating the spec?by introducing ad hock "out of spec" code to correct the error
4) perfectly acceptable error diagnostic


? IMHO 3) ?is a ?big gain - ?specifications always ?say what to ?do if
everything works ?- but never what ?to do if the ?input conditions are
not met - the usual answer is something sensible - but what you're the
programmer - In C etc. you ?have to write *something* if you detect an
error - ?in Erlang it's ?easy - don't ?even bother to write ?code that
checks for errors - "just let it crash".


? Then ?write a ?*independent* process ?that observes ?the ?crashes (a
linked process) - ?the independent process should try ?to correct the
error, if it can't correct ?the error it should crash (same principle)
- each monitor ?should try a ?simpler error recovery strategy ?- until
finally the ?error is ?fixed (this is ?the principle behind ?the error
recovery tree behaviour).


? Why was error handling designed like this?


? Easy - to make fault-tolerant ?systems you need TWO processors. ?You
can never ever make a fault tolerant system using just one processor -
because if that processor crashes you are scomblonked.


? One ?physical processor does ?the job ?- another ?separated physical
processor ?watches ?the first ?processor ?fixes ?errors ?if the ?first
processor ?crashes - this ?is the ?simplest possible ?was of ?making a
fault-tolerant system.


? This principle is mirrored exactly in the Erlang process structure -
this is because we want to have "location transparency" of processes -
in other ?words at a ?certain level of ?abstraction we do not ?wish to
know which physical processor an individual Erlang process runs on.


? This is ?the fundamental reason ?why we use "remote ?error recovery"
(i.e. handling ?the error ?in a different ?process, to the ?process in
which ?the error occurred) ?- it ?turns out ?that this ?has beneficial
implications for ?the design ?of a system; ?mainly because there ?is a
clean separation ?between doing a job, ?observing if the ?job was done
and fixing an error if an error has occurred.


? This ? organization ?corresponds ? nicely ?to ? a ? idealized ?human
organization of ?bosses and workers - ?bosses say what is ?to be done,
workers do stuff. ?Bosses do quality control and check that things get
done, if not they fire people ?re-organize and tell other people to do
the stuff. ?If ?they fail (the bosses) they get ?sacked etc. ?<<note I
said, idealized organization, usually ?if projects fail the bosses get
promoted and given more workers for their next project>>

? /Joe

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