Hacker News Reader

Comment by phoe-krk | Hacker News Reader

It is the latter.

It is possible to implement a condition system on top of any language that has dynamic variables, a `finally`-style construct and some mechanism for unwinding the stack. Since dynamic variables are implementable on top of lexical variables and `finally`, it's basically just about unwinding the stack and `finally`.

The main issue is how a condition system would fit with any existing system which likely works by immediately unwinding the stack rather than allowing to wind it further; that's the case e.g. in Java or C++.

View on HN

Isn't it possible to do in any system with CPS as well? I've not tried it but I would expect to be able to have a "conditions system" using the CPS monad in Haskell, for example. And Haskell doesn't even have variables the way most programmers think of them.

I haven't explored the CPS topic deeply, so I cannot really answer. As far as I understand, CPS preserves the state of the program or stack information by storing it in closures that are ready to be called at any time, whereas a condition system simply does not unwind the stack by simply not unwinding it and executing on top of the already existing stack, ready to both return control to the signaling code and to transfer it somewhere up the stack.

In its nature, a condition system is simply a means of executing code that has been provided dynamically, including transfers of control. I think that a closer term would be algebraic effects, which seem to be an equivalent of a condition system in a strictly typed strictly functional world.

CPS can be kind of thought of like a normal programming language with the modification that every function takes an additional parameter "the rest of the program" and calls this instead of "return". Of course once this mechanism is in your language you might have multiple "rest of the program" parameters which the function can pick between.

I found this old thread talking a bit about the lisp condition system [1] which also mentions implementing it in a typed manner.

I used Lisp for a while myself as my favourite language (the condition system was part of the reason) but the problem I ended up switching to Haskell because it has much of the power macros provide coupled with a very strong type system. That's why the potential occurred to me that CPS can probably implement something effectively equivalent condition system.

[1] http://lambda-the-ultimate.org/node/1544

I see. I know that it is possible to transform primitive CL control flow operators into CPS, as it is shown on https://gitlab.common-lisp.net/cl-cont/cl-cont/-/blob/master..., so I assume that it is also possible a condition system into CPS as a derivative of those operators and then possibly optimize it further to take CPS-specific code traits into account.

Reply on news.ycombinator.com