I'm working on a tokenizer for that involves some level of "self-healing":
if the input port contains an unexpected character, an error token is to be
emitted, but computation isn't over -- we are to continue by substituting
the bad character with a good one, emitting that, too (thereby recovering),
and keep rolling.
When the tokenizer encounters an unexpected character, what I'd like to do
is similar to raising/throwing an exception, but what's going on isn't (in
my understanding) quite the same as raising an exception, because I don't
want to just have some ambient exception handler deal with the error, since
my understanding is that exceptions don't have enough information to simply
resume the computation. I guess I'd like to install a prompt, but when
control goes back to the prompt, I'd like it to be possible to resume the
computation.
An old discussion here from 2015, with John Carmack ("continuing after a
user break"), comes close to what I have in mind. It's about setting up a
handler for breaks. I guess what I have in mind are breaks, but what's not
clear to me is how to raise them in my code. It seems like `break-thread`
is the only way to do that? The discussion of breaks in the docs involves a
lot of talk about threads and user interaction. But what I'm doing isn't
really about threads at all (I'm making a tokenizer, not a REPL or other
interactive program), so it leaves me feeling uncertain that I want breaks
after all. And even if breaks are a technically viable solution, I wonder
if there are any performance penalties or other gotchas that could be
avoided by using some other continuation forms.
Here's some pseudo-Racket that gets at what I'm looking for:
; compute a list of tokens
; -> (listof (char? or eof))
(define (handle-many)
(watch-for-break (lambda (e) (handle-break e))
(match (handle-one)
[(? eof-object?) '(eof)]
[(? char? c) (cons c (handle-one))])))
; -> char? or eof
(define (handle-one)
(define c (peek-char))
(match c
[(? eof-object?) eof]
[#\f
(break-with c) ; pass the unexpected character to the caller...
(read-char in) ; ...but after that, resume here!
#\a]
[else
(read-char in)
#\b]))
Any suggestions?
Thanks,
Jesse
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