I don't understand why this code behaves differently in different implementations:
(format t "asdf")
(setq var (read))
In CLISP it behaves as would be expected, with the prompt printed followed by the read, but in SBCL it reads, then outputs. I read a bit on the internet and changed it:
(format t "asdf")
(force-output t)
(setq var (read))
This, again, works fine in CLISP, but in SBCL it still reads, then outputs. I even tried separating it into another function:
(defun output (string)
(format t string)
(force-output t))
(output "asdf")
(setq var (read))
And it still reads, then outputs. Am I not using force-output correctly or is this just an idiosyncrasy of SBCL?
You need to use FINISH-OUTPUT.
In systems with buffered output streams, some output remains in the output buffer until the output buffer is full (then it will be automatically written to the destination) or the output buffer is explicity emptied.
Common Lisp has three functions for that:
FINISH-OUTPUT, attempts to ensure that all output is done and THEN returns.
FORCE-OUTPUT, starts the remaining output, but IMMEDIATELY returns and does NOT wait for all output being done.
CLEAR-OUTPUT, tries to delete any pending output.
Also the T in FORCE-OUTPUT and FORMAT are unfortunately not the same.
force-output / finish-output: T is *terminal-io* and NIL is *standard-output*
FORMAT: T is *standard-output*
this should work:
(format t "asdf")
(finish-output nil) ; note the NIL
(setq var (read))
Related
Sounds deceptively easy. This doesn't work:
~/.sbclrc
(load #P"~/in-package.lisp")
~/in-package.lisp
(in-package 'stored-package)
The package change only applies to the file in-package.lisp itself.
Try different approach: store just the name.
(defmacro recall-package (&optional (filename #p"~/lisp-package.lisp"))
"IN-PACKAGE the contents of the FILENAME"
(let ((p (car (uiop:read-file-lines filename))))
`(in-package ,p)))
This works, but only from ~/.sbclrc. Files which it LOADs expand the macro within their own context, and so it doesn't work.
SBCL reads it's .sbclrc like this:
(restart-case
(flet ((process-init-file (kind specified-pathname default-function)
(awhen (or specified-pathname (funcall default-function))
(with-open-file (stream (if specified-pathname
(parse-native-namestring it)
(pathname it))
:if-does-not-exist nil)
(cond (stream
(sb-fasl::call-with-load-bindings
(lambda (stream kind) (load-as-source stream :context kind))
stream kind stream))
(specified-pathname
(cerror "Ignore missing init file"
"The specified ~A file ~A was not found."
kind specified-pathname)))))))
(unless no-sysinit
(process-init-file "sysinit" sysinit *sysinit-pathname-function*))
(unless no-userinit
(process-init-file "userinit" userinit *userinit-pathname-function*))
Using these fancy sb-fasl::call-with-load-bindings and sb-int:load-as-source yields similar results to the above.
You can't do this with load, because
load binds *readtable* and *package* to the values they held before loading the file.
Function load
This means that any changes made to the values of these variables within a file being loaded are local to the file. That's almost always a good thing. In particular it means that there is no way at all (or no portable way: if you had access to the guts of the dynamic binding mechanism of the implementation this might not be true) that any changes made to the current package (ie the dynamic value of *package*) can ever propagate up through calls to load.
If all you want to do is set the package based on some name in a file, then this is relatively easy, with something like the below:
(defpackage :empty-package
(:use))
(defun set-package-from-file (f)
(let ((pn
(with-standard-io-syntax
;; EP just in case the file does somehow manage to smash
;; *package*)
(let* ((ep (find-package :empty-package))
(*package* ep)
(*read-eval* nil))
(unwind-protect
(with-open-file (in f)
(string (read in)))
;; Clean up EP to avoid leakage
(do-symbols (s ep)
(unintern s ep)))))))
(let ((p (find-package pn)))
(unless p (error "no package ~A" pn))
(setf *package* p))))
This is probably both overly-protective and thus will contain some horrible unexpected bug which I should have thought about (I know it's not safe against interning symbols in other packages). However the idea is that the file contains a single string-designator which should be the package name.
If you had time on your hands you could fairly easily write a version of load which would not rebind *package* &c, and which would work for source files. I think you can't portably write one which would work for FASL files.
Here's one reason why the behaviour the language specifies is the right behaviour: it makes compilation a lot easier. Consider a file which contains:
(in-package ...)
(defun foo (...) ...)
(load ...)
(defun bar (...)
(foo ...)
...)
If *package* could propagate up through load then compiling this file would be, at best, interesting.
I'm trying to write a formatting program for Common Lisp code, for which I need to tweak the behavior of the reader, e.g. with a reader macro for comments. Currently looking at #+ e.g.
(defun args ()
#+CCL CCL:*UNPROCESSED-COMMAND-LINE-ARGUMENTS*
#+SBCL (cdr *posix-argv*))
Default reader behavior is to discard the currently inactive branch entirely, but for my purposes I need to keep both. I think that means I need a reader macro for #+.
But # is also a prefix to lots of other things. How can I write a reader macro that handles #+ while keeping the default behavior for # everything else?
The # character is a dispatch macro character, meaning you can define a reader macro for the #+ combination without affecting any other reader macros beginning with hash.
You can start from the code below, and customize it to do what you want it to:
;; define the reader function
(defun custom-comment-reader-macro (stream char &optional num)
;; char will be #\+ in our case, and num will be nil.
;; stream will be the code input stream
(declare (ignore char num))
;; this is the default behaviour of #+, you can customize it below
(if (member (intern (string (read stream)) :keyword)
*features*)
(read stream)
(progn (read stream) ;ignore next token
(values)))) ;return nothing
;; tell the reader to use our function when it encounters #+
(set-dispatch-macro-character #\# #\+ #'custom-comment-reader-macro)
I was hoping to experiment with cl-async to run a series of external programs with a large combinations of command line arguments. However, I can't figure out how to read the stdout of the processes launched with as:spawn.
I would typically use uiop which makes it easy to capture the process output:
(let ((p (uiop:launch-program ... :output :stream)))
(do-something-else-until-p-is-done)
(format t "~a~%" (read-line (uiop:process-info-output p))))
I've tried both :output :pipe and :output :stream options to as:spawn and executing (as:process-output process-object) in my exit-callback shows the appropriate pipe or async-stream objects but I can't figure out how to read from them.
Can anyone with experience with this library tell how to accomplish this?
So you go to your repl and type:
CL-USER> (documentation 'as:spawn 'function)
And you read whatever comes out (or put your point on the symbol and hit C-c C-d f). If you read it you’ll see that the format for the :input, etc arguments is either :pipe, (:pipe args...), :stream, or (:stream args...) (or some other options). And that :stream behaves similarly to :pipe but gives output of a different type and that for details of args one should look at PIPE-CONNECT so you go and look up the documentation for that. Well it tells you what the options are but it isn’t very useful. What’s the documentation/description of PIPE or STREAM? Well it turns out that pipe is a class and a subclass of STREAMISH. What about PROCESS that’s a class too and it has slots (and accessors) for things like PROCESS-OUTPUT. So what is a good plan for how to figure out what to do next? Here’s a suggestion:
Spawn a long running process (like cat foo.txt -) with :output :stream :input :pipe say
Inspect the result (C-c C-v TAB)
Hopefully it’s an instance of PROCESS. What is it’s output? Inspect that
Hopefully the output is a Gray stream (ASYNC-STREAM). Get it into your repl and see what happens if you try to read from it?
And what about the input? See what type that has and what you can do with it
The above is all speculation. I’ve not tried running any of this but you should. Alternatively go look at the source code for the library. It’s already on your computer and if you can’t find it it’s on GitHub. There are only about half a dozen source files and they’re all small. Just read them and see what you can learn. Or go to the symbol you want to know about and hit M-. to jump straight to its definition. Then read the code. Then see if you can figure out what to do.
I found the answer in the test suite. The output stream can only be processed asynchronously via a read call-back. The following is simple example for posterity
(as:start-event-loop
(lambda ()
(let ((bytes (make-array 0 :element-type '(unsigned-byte 8))))
(as:spawn "./test.sh" '()
:exit-cb (lambda (proc exit-status term-signal)
(declare (ignore proc exit-status term-signal))
(format t "proc output:~%~a"
(babel:octets-to-string bytes)))
:output (list :stream
:read-cb (lambda (pipe stream)
(declare (ignore pipe))
(let ((buf (make-array 128 :element-type '(unsigned-byte 8))))
(loop for n = (read-sequence buf stream)
while (plusp n) do
(setf bytes
(concatenate '(vector (unsigned-byte 8))
bytes
(subseq buf 0 n)))))))))))
with
$ cat test.sh
#!/bin/bash
sleep_time=$((1+$RANDOM%10))
echo "Process $$ will sleep for $sleep_time"
sleep $sleep_time
echo "Process $$ exiting"
yields the expected output
It might seem simple, but I can't get it to work. I simply need to read a file where the contents are just one big list
(a b c d)
. . . as is . . . into a list in my program. I have
(let ((ardplst nil))
...
(with-open-file (in ardpfile :direction :input :if-does-not-exist nil)
(when in
(read-sequence ardplst in))
(format t "~a" ardplst))
But it's not working. I get NIL. What am I doing wrong?
What does read-sequence do? It reads some elements from the stream, typically characters (but it depends on the element-type of the stream) and destructively insert them into the input sequence. So, you would collect characters #\(, then #\a, then #\Space, then #\b, etc. However, reading stops as soon as you reach the end of your sequence: with your empty list, that means immediately (you are supposed to pass a buffer, e.g. a vector). In you case, read-sequence returns 0.
The reason you get nil is because your last expression is format, which in the above code outputs to the standard output (because of t) and returns nil. You could have used print, which returns the printed object.
I don't understand why you are explicitely using :if-does-not-exist nil. Are you sure you want to silently skip the task if the file cannot be opened? What if the list you read is empty? You should probably let an error be signaled in case the file is not found.
I would use read while disabling read-time evaluation:
(with-open-file (in my-file)
(let* ((*read-eval* nil)
(list (read in)))
(prog1 list
(check-type list list))))
Note that the default :direction is :input. In my opinion it does not hurt to omit this argument here, though sometimes it can be more readable to write it explicitely.
I'm trying to make a prompt for user input, but each time i call this function, instead of printing the ":", it waits until I press something and after that prints the character ":".
I can't find anything on the web.
(defun MovimientoAdversario ()
(let ((aux))
(format t "~% :")
(setf aux (read))))
Try flushing the output buffers before reading:
(format t "~% :")
(force-output)
(setf aux (read))