Today my goal is to learn about how to use continuations to implement a few of the following things in Racket:
- Exceptions (including
- Time-traveling search
- Non-deterministic programming
dynamic-wind is a function which takes three arguments:
post-thunk. It has the special property that even if
value-thunk returns early (from calling a continuation for example)
post-thunk will still be called.
It can be used to implement cleanup, such as closing a file, no matter what happens in
value-thunk. A great use case for
dynamic-wind is using the
post-thunk for cleanup code like closing a file once you’re done reading from it:
(define (safe-read-file input-file) (let ([p (open-input-file input-file)]) (dynamic-wind (lambda () '()) (lambda () '()) ; do something with the file here (lambda () (begin (close-input-port p)))))) ; this will always be called!
After reading through Matt Might’s article on implementing exceptions, I realized that I had to take a slight detour to study the
define-syntax procedure, which is a form of macro in Scheme.
define-syntax is how you define macros, or templates of code, which expand into more code! I don’t really fully understand this very well, but I hope to dive into this a bit more tomorrow.
I ended up not getting very far with learning how to implement exceptions in Racket (I had lots of chats with great people!) so here are some open questions I hope to answer tomorrow: