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Comment by jahewson | Hacker News Reader

> why don’t more languages have them?

My guess would be that they are not needed. Exceptions for anything other than program death are something of an anti-pattern, potentially spreading the internal details of some piece of code all the way up the stack.

My choice of implementation would be to pass a discriminator function which takes an error and returns true if the error is recoverable. Then you have the choice of either returning all the errors at the end or reporting each earlier when the discriminator is called.

Edit: That's not to say that exceptions can't be used effectively, they just mostly aren't.

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> My choice of implementation would be to pass a discriminator function which takes an error and returns true if the error is recoverable.

Conditions are close to exactly that + syntactic sugar.

`handler-bind` lets you establish a dynamic scope, inside of which specific functions are called when specific conditions are signaled. Signaling a condition = call the first handler function in the list that handles the signaled condition. The handler is called before the stack is unwound, which allows it to elect to either `throw`, invoke a restart, or ignore the condition entirely.

This is in contrast to exceptions in (say) Java. `throw new FooException()` does a lot of potentially destructive work before control gets transfered to the exception handler: It allocates an exception object, which fully captures the current thread's stack. (It's not too common, but this can be bypassed, if you don't throw a new exception object...) It unwinds the stack to the frame that has a matching `catch`.

Only after those two things have happened does the exception handler get a chance to decide how to proceed, and at that point, it's too late for many recovery strategies. Conditions help this by making it easier to intercept the process earlier.

Where they hurt is that they add another level of complexity to API design. With Java style exceptions, if the function fails, it fails completely. There's a convenient simplicity to that. With Lisp style conditions and restarts, A function will let you know how things are going as it does its work and then give you the opportunity to change its behavior midstream. It's a lot more work to design that kind of API, and it's probably well past the point of diminishing returns for most kinds of development.

Great answer. I'm left wondering if the solution is more complex than the problem.

There might be two different perspectives on this:

* automated error handling, mostly invisible to the user. In this case the condition system may give an attractive architecture, but it is not widely explored and it is not clear how it would integrate into a language like Java. There could be a benefit, but it is not explored by software architects. Apple's Dylan probably had something to it, but it was not widely used.

* error handling in interactive systems. That's where the Lisp community used/uses it mostly. The largest effort to use the condition system were in the OS of the MIT Lisp machine descendants - especially the one from Symbolics. It was used both in the development environment - which is tightly integrated in the OS. Most of the development tools were using it. But things like the networking code or the file system were also using it. The user interface is generally slightly more complex than current graphical user interfaces and it was expected that the user used this kind of power. When you get a dialog of restarts presented, the user needs more thinking about how to proceed than with a simple cancel/abort dialog box. There were also applications written using it and for that slightly simpler interfaces were used. For example the restarts were presented in dialogs and not in a debugger context.

It takes a bit of re-orientation to imagine a user interface, where the user has the option to repair/retry a failed operation, instead of the usual abort/redo. These benefits are easier to get in an integrated system (-> Lisp, Smalltalk) - a model where several non-integrated tools interact in a condition system is more difficult to imagine.

Today, most of the better Common Lisp systems are using the condition system in their environment. There the power can be used, but at the same time it is also easy to ignore and just use a default mode of using the abort restart - without getting the benefits of thinking a bit more and investing the time to use a more complex restart.

In practice the option to resume is used much less frequently than the option to throw. There are two primitives one can use to signal a condition: SIGNAL allows resumption, but ERROR does not. So if you don't want to deal with the complexities of a possible resumption, simply call ERROR.

Similarly, the client, to establish the handler, can use HANDLER-BIND, which is the fully general form that permits resumption, or HANDLER-CASE, which is simpler to use but does not permit resumption.

In practice, most of the time people use ERROR and HANDLER-CASE, and so the extra complexity does not intrude at all.

OTOH it is occasionally handy to be able to resume.

My understanding of Seibel is that Lisp's condition system is designed to avoid exactly the problem of internal details spreading up the stack.

It does so by allowing conditions (of which exceptions are one type) to be handled by a higher level of the call stack without unwinding the call stack, and allows for the determination of which particular condition handler to be made independently (and at a higher level of the call stack) than the implementation of the handler.

The "anti-pattern" may be a result of the way in which other languages implementation of exception (or condition) handling differs from that of Lisp.

> [conditions are] handled by a higher level of the call stack without unwinding the call stack, and allow for the determination of which particular condition handler to be made independently (and at a higher level of the call stack) than the implementation of the handler.

Is exactly what the higher-order function I described would let you do. Why do we need conditions?

Edit: the answer to this question is "because we want to use exceptions". But there's really no need to.

Per Greenspun's Tenth Rule you do not.

http://paulgraham.com/quotes.html

> Is exactly what the higher-order function I described would let you do. Why do we need conditions?

Because conditions let you do that correctly without e.g. changing all intermediate pieces of code so they forward some sort of gigantic mapping of callables through the stack.

That's non-essential, but certainly nice to have. Thanks for the insight.

That is, of course, the standard argument for dynamic binding in general.

So in CL one could certainly pass a callback via dynamic binding and get the same effect.

The choice to use the condition primitives instead is, then, simply a stylistic one.

For those who wonder whether that is worth a new idiom to learn: the stylistic effect is enormous.

For example, there could be 5 layers of code between a condition handler and the code signaling a "file not found" condition, and the top levels of that stack might not even be aware that they could trigger such a condition or even any condition at all.

In some sense, this is the reverse of Java's checked exceptions. There, you have the problem that an interface declares exception X and Y, and an implementation that wants to throw a Z has to encapsulate that exception into a X or an Y. Here, the intermediate layers _can_ be blightfully unaware of those conditions.

A common use case is to use conditions without registered handlers as "break on exception" breakpoints, with the benefit that the human working with the debugger has more options than "stop execution", "continue and pray" and "tweak some memory, set the PC, go, and hope for the best". For example, if the 'open file' system call has a restart, users can easily fix "oops, that lengthy computation is done, but I mistyped that output file name." error conditions.

A possible benefit of conditions in Common Lisp is more than 25 years worth of implementation experience and full documentation of how the system is structured.

Extensive literature about the language is a defining feature of Lisp.

Conditionals as detailed in Common Lisp the Language, 2nd Edition: http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node312.html#S...

Hmm, the same argument could be made of GOTOs in FORTRAN.

GOTO's use in Fortran has continued to be deprecated for several decades.

One might say that they are not Fortrany in the way that conditionals are Lispy.

Lisp's condition system allows you to handle the condition without unwinding the stack.

That's the whole point.

So does a higher-order function...

Therefore it can't be the whole point, because that mechanism already exists, which most commenters here seem to have not realised.

The actual point is, that it gives you more powerful exceptions, but you don't actually need them.

Not the same, because you need to pass function to be called on error throught all the stack, and you don't need to do it with conditions, if I understand correctly.

Example: main calls calculateSth calls f1 calls f2 calls f3 calls readFiles

If we want to decide in main how we want to react to errors when reading files, we need to pass the function to handle errors thorught the whole chain.

With conditions calculateSth,f1,f2 and f3 don't need to know about our condition.

This also means that when you add new condition to function that you invoke in many places, you need much less changes everywhere.

At last, an actual answer, thanks. Basically, we can save the need to pass a callback argument.

And the potential need to pass way more than one, if you have 5 different error conditions which can exist in the same stack you need to come up with some sort of standard way to express that (e.g. a mapping of condition to callable). Then you need to find out how to express inheritances/overrides, and the like.

Nah, you only need one error callback with a parameter, i.e. `f(error)`. You could override this at any point using closures. We're talking almost identical code.

And you also define the recovery strategies within the function that cares about them. It makes the code much more readable.

You also continue the computation from that call-site without losing the stack. If after a particular restart is invoked, processing can continue along the chain without missing a beat.

No... we could already do that with higher-order functions. We're going backwards here, please read the previous comments, specifically from ajuc.

As stated, a higher order function gives you all of these features already. (The discriminator function I suggested would just return a bool to indicate whether or not error recovery should occuur, which is handled at the call site, in the appropriate context, with the appropriate stack).

All Condition Handling brings to the table is the ability to omit passing the callback around. All the other features were already there.

Edit: mschaef's comment confirms this.

It's not even that, this can be (and probably is) implemented with dynamic variables (or some similar mechanism). You can also implement exceptions with setjmp and longjmp in C.

I feel the most important thing is conventions, and having a weaker model of exceptions as a convention, affects all/most code in the platform. In Java most libraries throw exceptions and so you will probably use exceptions. In C most libraries return an int return value for error/success and you'll probably use that. In Common Lisp, people use conditions.

So, what I'm trying to say, is that when considering this kind of design decisions in a language, we have to take into account how it affects the whole platform (and not just see if it is possible to implement it any other way in our particular language of choice). The same reasoning should be applied for object orientated features, scoping rules, calling conventions, multiple-value returns, lazy evaluation, macros, continuations, modules, type systems, naming conventions, etc.

This reductionist philosophy is mostly inappropriate when talking about programming language features.

Yes, from what I understand about Conditions (IANACL), they can be implemented with first class functions and dynamic variables. And dynamic variables can be implemented by passing an extra argument to every function. And function arguments can be implemented with well-named global variables. And functions can be implemented with computed gotos and a reified global stack variable.

But the point of a language is to create a common vocabulary with which programmers can think and talk to machines and each other. Try to reduce everything to a simpler form and everyone ends up reimplementing the higher level concepts in incomplete or at least incompatible ways with everyone else. There is a need for experimentation and turnover to figure out which higher level constructs are best, but this is entering the realm of language design and should not be a required part of application programming.

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