moving general-purpose functions from httpd_util to stdlib

Robert Virding robert.virding@REDACTED
Tue Apr 15 02:12:39 CEST 2003

----- Original Message ----- 
From: "Bengt Kleberg" <eleberg@REDACTED>
To: <erlang-questions@REDACTED>
Sent: Monday, April 14, 2003 8:08 AM
Subject: Re: moving general-purpose functions from httpd_util to stdlib

> > Date: Mon, 14 Apr 2003 00:01:55 -0500
> > From: Chris Pressey <cpressey@REDACTED>
> ...deleted
> > This brings up one of those burning questions of our times - that is,
> > what exactly "defensive programming" means in the context of Erlang.
> > 
> > Say you have a function foo() that can return either an integer or the
> > atom 'error'.
> an alternative to this problem is described by Richard Carlsson:
> - A function should *not* return wrapped values like
>   {ok,Value}/{error,Reason} to indicate success or
>   failure. The assumed behaviour should be success,
>   and failures should be signalled by exceptions,
>   as described below.

I think I would try to qualify that a little more. It depends on what type
of failure you mean. I would generally say that bad argument type
or values should generate an exit so it is quite clear that Chris's
hex conversion functions should only return the value and exit on
failure. However, there are many cases where the non-success
case does not really represent an error and then it should not
generate an exit but use qualified return values like
{ok,V}/{error,R}. Maybe the convention to use ok/error was badly
chosen and we should have used yes/no but I think it would be
worse to try and have two, or more, "conventions". Better to
have one and try to explain how it is used.

This is why most of the original, older libraries at least very rarely
check arguments types and values but just assume they are
correct and otherwise exit. As do most bifs. You can't do anything
sensible if the arguments are all wrong.

Another example would be file:open. As I envisaged when I wrote
it was that if the arguments were of the wrong type or had illegal
values ('creat') then it should exit. However, not being able to
open the file I didn't really see as an error so it returned qualified
values to indicate this. What it exactly does now I don't know.

Not everyone agreed with me.

Basically there are errors and errors.


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