Commit ada5cacb authored by David Byers's avatar David Byers
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Updates :-) Don't remember what

parent 5f2632c1
......@@ -186,5 +186,5 @@
David Byers
This file explains some traps you may fall prey to when writing
server callbacks. All developers must know this.
Da Rulez:
* Never rely on dynamic scoping in callbacks
* Use collectors to pass results from a callback to a waiting
* Don't use lexical-let or closure.
* Did I say never to rely on dynamic scoping?
Sometimes you might want to use dynamic scoping to access
variables in the environment where you specify a callback.
Typically the code looks something like this:
(let ((foo nil))
(initiate-get-text 'main (lambda (x) (setq foo x)) text-no)
(lyskom-wait-queue 'main)
(use foo))
This is unsafe. If the user interrupts the function (C-g) after
the call has been made to the server but before the reply has been
received (not unusual when the server is slow to respond), the
environment in which foo is bound no longer exists. When the
callback is eventually called, it will access or bind foo in a
different environment than was intended. If you're really unlucky
the user may have initiated some other function that uses a
variable named foo, and when the callback is executed it will not
only use a variable in the wrong environment, but it may set that
variable to something that confuses the code in the new
The solution is to either implement static scoping for Emacs,
which would make you something of a hero, or to avoid the problem
entirely. The collector data type is intended for this
application. The code above can be written using collectors:
(let ((foo (make-collector)))
(initiate-get-text 'main (lambda (x res)
(set-collector->value res x))
(lyskom-wait-queue 'main)
(princ (collector->value foo)))
A collector is really just a cons pair where the cdr is the value
of the collector. The caller passes the collector (the value of
foo) to the function that eventually causes the callback to be
called. The callback only references its parameters, not the
caller's environment. The caller finally extracts the value from
the collector.
If the function above gets interrupted, the cons cell still exists
even if the binding to foo is lost. The callback will set the
value of the collector and exit. Eventually the collector will be
garbage collected since there are no longer any references to it.
When playing with collectors, these are the functions you might
want to use:
make-collector Create a new collector.
collector->value Return the value of a collector.
set-collector->value Set the value of a collector.
collector-push Push a value onto the front of the
collecor's value. More or less the
same as (set-collector->value
collector (cons value
(collector->value collector))
set-value-of-collector Set the value of a collector.
The functions collector-push and set-value-of-collector can be
used as callbacks. If you are making lots of server calls and
collecting the results in a list, just use collector-push as the
callback for your server calls, and pass the collector as an extra
argument to the initiate-* function. If you are making a single
server call and want the result in a collector, use the
set-value-of-collector function as your callback.
At this point you may be thinking "why not use lexical-let or
petli's closure". I'll tell you why.
First, lexical-let is something that comes with cl.el. This
package is not in the standard install of Emacs 19.34, so don't
use it. And even if you could use it there are reasons not to:
* It rewrites the code, replacing all references to lexical
variables with references to gensymmed variables. This sort of
ensures unique names. But what if you interrupt the function,
then call it again. The new environment will be using the same
names as the old one (the names are generated when the function
is defined), so both the new and the old callbacks will affect
the same variables.
* The code rewriting is buggy. Try expanding the following snippet
and you'll see what I mean:
(defmacro fie (x) (list 'quote x))
(macroexpand '(lexical-let ((foo nil)) (fie (setq foo t))))
If all was well, lexical-let wouldn't touch the setq or the foo,
because the call to fie will cause all of that to be quoted. But
lexical let rewrites that part too, causing the meaning of the
code to change. If the rewriting fails on one easy example (when
testing lexical-let this was the first test I tried) I do not
want to trust it elsewhere.
There is another package, closure, written by petli (who has
hacked some of the client). The rewriting process has different
problems from lexical-let: it gets the example above right but
fails to terminate on other inputs. It has the same variable
naming problem that lexical-let has, which is enough to disqualify
it. It also requires you to use setq to set your variables, which
may not always be what you want. Just as with cl, closure is not
part of your default Emacs.
Finally, the lexical-lets will only help you when your callback is
declared in-line. If it's in a different function, you actually
*want* dynamic scope, sort of. Use collectors instead.
Da Rulez:
* Callbacks must never, ever, make blocking calls to the server.
* Callbacks must never, ever, interact with the user.
* Callbacks must never, ever, call accept-process-output
The parser is not reentrant.
Your callbacks get called before the parser has cleaned up after
the call that caused your callback to be called. If you cause the
parser to be called recursively in this state, things will break.
You can cause the parser to get called recursively by calling
anything that in turn causes process output to be accepted. This
includes, but is not limited to, interacting with the user through
read-from-minibuffer, calling accept-process-output, calling
lyskom-wait-queue (or any other function that waits until some
server call has completed) and calling blocking-do.
If you make a recursive call to the parser it will detect this and
present the user with a stack trace.
The parser could be fixed to be reentrant (you'll also have to
deal with the parsing of asynchronous messages in async.el), but
even if it were reentrant, making blocking calls within callbacks
would be a pretty bad idea since it might result in a slower
Asyncronous calls (the initiate-something calls) are prefectly OK
since your function will return (and the parser will return)
before the parser gets called again.
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