<RavenJoad>
Hey all, I am facing an issue with string formatting and outputting a string to a stream. The following is the minimum to reproduce this. (let ((stream 'nil)) (format stream "~a" "#(") (format stream "~a" ")")) I expect "#()" to be the final string, but SBCL return just ")", losing the leading "#(".
<RavenJoad>
Oh... As soon as I post the question, I solve it. Only the final string was being returned from the progn. Duh. I needed a uiop:strcat to make this work.
<aeth>
because format nil turns it into returning a string instead of writing to a stream
<aeth>
use with-output-to-string stream instead of nil
<RavenJoad>
aeth: I should rewrite the function to use that, for sure. That would make the block easier. Inside a with-output-to-string form, what should the destination of FORMAT be?
<RavenJoad>
Right. I need to more closely read CLHS before asking these questions. I'm doing code-gen and want to have the final destination be a string for unit testing and a file for actual output, so I guess I accidentally hacked together the same macro.
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<RavenJoad>
I guess what I really want is with-output-to-stream, so I can plug in either *standard-output* or a file stream.
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<RavenJoad>
Does such a macro exist? I cannot seem to find one. I can do (princ (with-output-to-string ...) stream), but that always produces something on *standard-output*, which I don't want.
<pranav>
RavenJoad: FORMAT can write formatted output to a given stream.
<shawnw>
Look into UIOP's with-output and friends.
<RavenJoad>
I think I may have an XY problem/desire here. I want my method to explicitly accept a stream, and produce the contents to that stream. Unit tests need this stream to be a string buffer, actual usage requires this stream to be connected to a file.
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<Noisytoot>
,(let ((*standard-output* (make-string-output-stream))) (format t "a") (format t "b") (get-output-stream-string *standard-output*))
<ixelp>
=> "ab"
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<Noisytoot>
RavenJoad: like that?
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<RavenJoad>
Noisytoot: I realized I XY-d myself into a writing format. I removed a uiop:strcat and have my unit tests use (uiop:with-output (str 'nil) (generate str)), which does what I want. I forgot I wrote a helper function to make calling generate easier in tests.
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<RavenJoad>
I'm using trivia, and want to match against lists of either 1 or 2 elements, and throw an error otherwise. Reading the documentation does not show an "else" or
<RavenJoad>
't condition that I can use. Am I just missing something?
<char>
RavenJoad: (_ ...) is the else case for trivia:match.
<RavenJoad>
char: Perhaps I am missing something, but (trivia:lambda-match ((_ ...) (print "this is an error"))) fails to compile due to "too many dots".
<char>
the dots are meant to be your code.
<|3b|>
there is also ematch if you don't care about the specific error
<char>
(trivia:lambda-match (_ (print "this is an error")))
<|3b|>
and OTHERWISE if you want something that looks more like CASE etc
<RavenJoad>
Whoops. I am too used to Scheme/Racket syntax. I thought it was a special thing here like over there.
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<alcor>
rainthree: Regarding point #3, I believe this is why the *mufflers* convention is used for naming dynamic variables, so the risk of accidential rebdinding is reduced.
<|3b|>
it is, and #3 notes that the ** convention is a sign there is a problem
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<alcor>
I've also seen people use +constant-variable-name+ for constants, to indicate that the variable is (semantically) used as a constant and should not be rebound.
* |3b|
agrees that there should be separate namespaces for constants and specials
<|3b|>
++ is more intended to indicate the variable was bound with DEFCONSTANT and that attempting to rebind it is undefined behavior. indicating it is semantically constant is just a derivative of that
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<alcor>
I have an issue with the way defconstant is implemented in some CL implementations, actually – it impedes my "live coding" workflow where I write code do the "eval as you go" thing, evaluating definitions of the program while writing it – Doing SLIME's `slime-eval-buffer' on a buffer with defconstant in it results in an error condition being raised (at least with SBCL) :(
<|3b|>
would you rather it leave old code using the old value without telling you?
<alcor>
Of course not, but I wish there was a way to just reevaluate the defconstant form without it complaining. I mean explicitly, on user request.
<|3b|>
you can teach slime how to do that if you really want to, if picking the "do it anyway" restart is too much work
<alcor>
Something like a hypothetical M-x slime-force-eval-buffer would be useful in this case, yes.
<|3b|>
or just use defparameter until you think the value is stable :)
<alcor>
Well, yes, that's what I usually do now.
<alcor>
Either that, or I elect to put the constant in a separate file.
<|3b|>
also, in cases where you didn't actually intend to change the value, but the value is one that isn't EQL on reloads, consider alexandria:define-constant or similar
<|3b|>
defconstant doesn't complain if the value didn't change
<alcor>
Yes, for simple constants everything is fine. It's only annoying if the value is a class instance.
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<|3b|>
right, use alexandria:define-constant or similar for that
<alcor>
I see. alexandria:define-constant allows one to specify the equality predicate. That is indeed a more elegant solution, thanks |3b|.
<alcor>
Regarding special variables: It's hard to find resources related to the performance implications of using them. My mental model the abstraction const is that they are equivalent to a thread-local variable, but I'm not entirely sure.
<|3b|>
they pretty much are thread-local variables if threads are available on all the CL i'm aware of
<|3b|>
they have an extra indirection compared to lexical variables, since they usually have to actually exist, and they also usually need extra work to remove the binding, which might prevent tail-call elimination
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<|3b|>
theoretically a compiler could figure out that a particular binding can never be accessed non-locally and make it as fast as a lexical, but i don't know if anyone actually tries to do so
<alcor>
It's been a while since I used thread-local storage in C & C++, but I remember there was a subtle footgun with threadlocals in the sense that they
<|3b|>
particularly since anything that errors could allow a handler to try to access it
<alcor>
… are lazily initialized, and the initialization is protected with a mutex.
<|3b|>
(and that is before considering things like interactive debugging)
<bike>
specials won't have that, since if they have global bindings that's not initialized per thread or whatever, they're actually global
<bike>
of course there's the other footgun in that accessing them concurrently could be a race, but at least it won't be a slow one!
<|3b|>
yeah, i wouldn't expect them to match any particular implementation of thread-local variables in other languages/runtimes
* alcor
tries to dig into SBCL's internals
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<alcor>
https://github.com/sbcl/sbcl/blob/master/doc/internals/specials.texinfo – "With multithreading it's slightly more complicated. The symbol's value slot contains the global value and each symbol has a TLS-INDEX slot that - when it's first bound - is set to a unique index of the thread local area reserved for this purpose."
<ixelp>
sbcl/doc/internals/specials.texinfo at master · sbcl/sbcl · GitHub
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<alcor>
So yes, it does use TLS, but no mutexes are involved fortunately.
<bike>
cos there's no way to specify a thread local initializer
<bike>
well
<bike>
i guess that's what that default thing is for actually, but those are run when the thread starts
<bike>
probably since they're specified per-thread, unlike thread-locals in C and C++.
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<lagash>
I finally got my hands on Common Lisp Recipes, and was not expecting so large of a book!
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<bjorkintosh>
lagash: how disappointed would you be if it were a mere 40 pages long?
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