First let me apologize for the length, second I will say that to my knowledge none of this is about the CL condition system in particular, third this is basically a brain dump because I need to get back to work and still don't understand the error code camps points...
I definitely wasn't trying to insult but I recognize that I wasn't being helpful either. I was basically throwing up my hands. And while I like CL's system, I haven't come close to trying everything out there so I can't be a judge; I mean this not like a cop out but seriously, I am very poorly informed about what other languages have to offer.
But since you ask: I guess if I was trying to convince someone that exceptions are inherently a better mechanism, I would start by exploring how arithmetic is handled in the C language. Let's say we have a function called "add" that adds two numbers. Typically we would think expect that the return value of such a function should hold the result of that addition. It would be naive, however, to expect add to always complete without something like an error happening. While addition of two numbers seems safe, it isn't when we start using our imperfect number representations such as floating point numbers (susceptible to overflow, underflow, loss of precision, and more) but also with machine sized integers (susceptible to overflow) and even bigints (what happens if we attempt to add two numbers and exhaust system heap space?). The way these are currently handled in the C language (to the best of my knowledge) is by three different mechanisms: 1) floats return special floats that say indicate that there was an error somewhere, like NaN (though you can probably instruct your OS/hardware/whatever to issue these as signals instead of silently spitting out an NaN), 2) wrap-around is loosely taken as a standard but truly it has undefined consequences, your code must guarantee that this cannot happen, and 3) the program will probably exit. This is messy but we have learned to deal with it and has become second nature to C programmers; but that doesn't means it is good.
But this doesn't need to be the case, we could use the return value checking that is being promulgated by some here. We can define add as...
error_code add(int a, int b, int *return)
...where int could be any number type and translate every occurrence of "a+b" in our code into something like...
int ret;
if (err = add(a,b,&ret))
{
// err handling here
}
// ret holds the answer here
I cannot believe this is preferable to anybody, in fact I am going to go out on a limb and say that it isn't. Instead we just trust this to work when we know that there are instances where it won't and, in the case of floating point math, it is extremely common for you to hit those cases where it doesn't work. If we were serious about writing code that handled these errors gracefully, our C code would be unmaintainable. If we were honest about the source of this laziness, I think we would say that this is due to the syntactic and mental overhead of the error handling. The fact that we don't code like this, even the very fact that C will spit out an NaN and propagate it along indefinitely, is proof in point that people dislike this type of "check the return value" error handling. This is C more or less pushing us to write code that is less robust.
If we really wanted to consider what happens in a Common Lisp system, you would have a function "add" that takes two numbers and will always return the correct result of that addition or won't return at all. This means no wrap around overflow, no NaNs; this is vastly cleaner. However, as far as I know, this has nothing to do with CL in particular, this is presumably how Python or any language with a exceptions works. This is what I cannot fathom: that there are people that are willing to throw away this simplifying assumption because it means that your code might have controlled non-local transfer of execution. This is what Joel on Software directly says (linked elsewhere in this thread); exception handling is like goto, goto is bad, even when it is contained within an apparatus fixes its problems, like the exception handling apparatus. Or that people think the same amount of work is involved in either system? How does that make sense? Which way would you rather treat something like "add(a, add(b, add(c, d)))" because to my eye the C style handling adds a bunch of boiler plate code that I'd rather not write while adding little to no value. Like I was saying, I cannot fathom it so it is hard to form an argument against the alternative stance that I do not understand. Perhaps there are places where we want our execution so close to the structure of the code that handling errors C style makes sense, but I have to imagine that they are few and far between.
If we compare a more common example where people will place error handlers: writing to a file. Is it better to have writefile(file, data) return an int that indicates error, or is it better to just say "writefile puts this data in file, period". Here are two errors that might come up when writing to a file, we don't have write permission for that file or the disk is full. Where do you want to handle these errors? One error happens when we open it, the other happens when we attempt to write to it further down the call tree, but presumably we might want to catch both and deal with them at the same location. With exceptions you catch/handle the exceptions you are interested in. When returning error codes you have to manually defer the error up the call tree, affecting the interface of every calling function up to the highest function that can actually handle the error. So you can add the boiler plate code to combine and propagate errors up the call tree to your task list now. At the root of this is the exact same issue as the previous point, it adds syntactic and mental overhead for the programmer.
This article seems to have spawned some responses. Consider this post:
http://www.yosefk.com/blog/error-codes-vs-exceptions-critica...
He makes claims like in the code...
open_the_gate()
wait_for_our_men_to_come_in()
close_the_gate()
...if there is an exception inside wait_for_our_men_to_come_in then we'll never close the gate. While this is true for the code he's written, most people would (or should) consider that code buggy. In my experience and the comments of that post confirm it, most languages have a mechanism to ensure certain side effects happen even under non-local control transfer. Where are the points of intermediate state that he render exceptions equally bad as error codes? Are they the states that we explicitly account for in any bug free program, like closing the gate if there is an problem bringing the men in? I will reiterate, I truly am at a loss, this is not a rhetorical question. It seems like I am missing something.
To sum up: One method seems like a clean method of dealing with errors that will happen, the other seems like a nightmare syntactically and basically consists of code that I deeply feel should be up to a compiler to write.