Condition Systems in an Exceptional Language - Chris Houser
ClojureTV1 HN recommender · recommended in 1 month, 2016–2022
talk · advanced · evergreen · 2016–2022 on HN
Chris Houser's Clojure/conj talk on why Clojure's exceptions destroy information as the stack unwinds, unlike Common Lisp's condition system, which can resume from the error site. He surveys the existing libraries that approximate conditions, explains why the…
Clojure commenters cite it as the best overview of condition systems in Clojure and of the libraries that attempt them.
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5 comments from Hacker News
There are some libraries for this. A great overview of the problem and some of the libs is ▶ this video and a good set of corresponding notes at link
True, the CL REPL is more useful than the Clojure REPL, but CL has a 20-30 year head start in development. Plus, Clojure was developed as a hosted language, and runs on .NET, Javascript, JVM, and even Dart (in-progress). Since exceptions are quite deeply integrated into those platforms (as opposed to conditions), it has "bled through" to the Clojure APIs, and, in turn, REPL. Folks have written a CL-style condition/restart library for Clojure ( link ) too. ▶ this video video too.
You can do that yourself if you care ( ▶ this video ) but I get what you mean. On the other hand, on a server application a REPL on exception wouldn’t help me.
It turns out that it is, in fact, at least mostly possible to implement a condition type system in Clojure. See ▶ this video I did a little bit of googling for Clojure libraries that do this: link and link I don't actually recommend any of these and haven't tried them. In particular I think using a non-standard library for your error handling, which is already probably relatively poorly tested, is probably a bad idea. I just wanted to point out that it's possible. I also think I remember something in a Rich Hickey talk (maybe "Clojure for Lisp Programmers"?) where someone in the audience asks about condition systems, and he responds that Clojure doesn't have one, but that it should be possible to build it.
The stack unwinding was such a sad thing to me. Reading about CL condition system made me smile widely. You have more data to treat the exception. While reading the abstract, suddenly the word Exception felt absurd. It's rarely an exceptional condition, it's really a parallel planned error checking. Also, being from the Java era, an Exception was mostly a boring way to ensure error checking, while lispers did use exception as cut / tree jmp to provide solution values to an algorithm. ps: funny today youtube suggested this ▶ this video