Re:Ask HN: Will Rust ever become a mainstream systems progra...
by lokedhs on Tuesday April 11, 2017 @05:37AM
(#14086157)
I'd recommend taking a look at Common Lisps condition system, which does exceptions right.
The most important feature is that catch blocks are executed before the stack is unwond, providing the ability for a handler to signal back to the context where the error occurred how it should be handled.
It's sad that no other languages implement this and instead simply decide that exceptions are bad.
Re:Ask HN: Will Rust ever become a mainstream systems progra...
by steveklabnik on Tuesday April 11, 2017 @09:11AM
(#14087349)
Fun trivia: Rust had conditions! Nobody used them, so they were removed.
Re:Ask HN: Will Rust ever become a mainstream systems progra...
by lokedhs on Sunday April 16, 2017 @11:39PM
(#14129086)
That sounds short-sighted to me. Even in Lisp restarts (i.e. the feature that allows a handler to communicate back to the point where the exception was thrown) aren't used that often, but when they are, they are incredibly valuable.
Re:Ask HN: Will Rust ever become a mainstream systems progra...
by kazinator on Sunday April 16, 2017 @11:51PM
(#14129116)
In TXR Lisp, a continuable exception occurs every time you see a warning about something in the code, like an unbound variable. It is intercepted higher up and automatically continued.
The code expander has an entry point which allows the caller to be informed about what variables and functions are free in a block of code. The implementation of this entry point works by intercepting warning exceptions about unbound symbols, and accumulating them in in lists, which are then returned to the caller.
This is a useful thing which reduces the need for users to write their own code walker in certain kinds of advanced macros.
This works fine if it's just a matter of rolling back a database transaction or releasing file handles (b/c someone else is doing all the work for you). But if you have more complicated invariants, your try catch has to handle restoring those invariants, no matter what path your code took.