Gambit-list Août 2005

gambit-list@iro.umontreal.ca

12 participants
35 discussions

refcounts and threads
by Christian 19 Aoû '05

19 Aoû '05

Hello I've noticed that my perl interface leaked memory until I added a ___still_obj_refcount_dec call after allocation of a scheme object. This is in code which converts a perl datastructure to a scheme one; the resulting object is not used by C or perl code, only by scheme. So since a newly allocated still object starts with a refcount of 1, I have to decrement it again. I now do this immediately after allocation, e.g.: ___SCMOBJ /* for ___err */ gperl__SV_to_string_SCMOBJ (SV *sv, ___SCMOBJ* obj) { ... result = ___alloc_scmobj (___sSTRING, n<<___LCS, ___STILL); ___still_obj_refcount_dec(result); // now copy over perl string, then: *obj = result; return ___FIX(___NO_ERR); } I use the above from scheme like: (c-define-type SV* (pointer "SV" SV* "gperl_release_sv")) (c-define-type SV*->string "SV*" "SV_to_string_SCMOBJ" "NOTIMPLEMENTED" #f) (define sv->string (c-lambda (SV*) SV*->string "___result=___arg1;")) My question: is the above thread safe? Do I have to add (declare (not interrupts-enabled)), like: (define (sv->string sv) (declare (not interrupts-enabled)) ((c-lambda (SV*) SV*->string "___result=___arg1;") sv)) Thanks Christian.

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macro troubles
by Alex Shinn 19 Aoû '05

19 Aoû '05

Gambit Version 4.0 beta 14 > (load "~~/syntax-case") "/usr/local/Gambit-C/syntax-case.scm" > (define-macro (foo . o) #f) *** ERROR -- invalid syntax (foo . o) Also, (include "macros.scm") will splice the forms in as the documents say, but only macros in that file will be available, and you'll get an error if they use any procedures defined in the same file. -- Alex

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RE: [gambit-list] Newbie: define-macro question
by Logan, Patrick D 18 Aoû '05

18 Aoû '05

> 2. Making little sub-languages for specialized processing e.g. > > (with-vectors (v1 v2 v3) (v1 <- (+ v2 v3))) > > for summing up the vectors v2 and v3 and storing it in v1. Nice! I also liked the challenge in example #1 of allowing multiple bodies in the bindings. Here's a challenge for #2 -- would this language be any more usable by eliminating the need to declare the vectors, i.e. how about this... (with-vectors (v1 <- (+ v2 v3))) -Patrick

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test driven development
by Thomas Hafner 18 Aoû '05

18 Aoû '05

Are there any tools helping me doing TDD using Gambit-C? I've found Neil Van Dyke's Testeez. But it doesn't seem to support stubs. I think there is the need, that a function under test may call a stub (belonging to the test environment) rather than the original function. E.g. it may be a good idea to stub read-line, otherwise the test wouldn't be automatical. I can even think about situations where it may usefull to stub functions which are part of the software to be tested, at least for a part of the tests: e.g. if the original function isn't stateless and it's difficult to get to a rare state. Then a stub representing that state is easier to deal with. Maybe I shouln't look for tools, but on ways how to write code, that can be tested easier. Programming in a functional way seems to be better there, but what to do then with non-functional functions like read-line? (I call it ,,non-functional`` because it can return different values although the argument list keeps being the same, i.e. the emptry one.) Just an idea: problematic functions (like read-line above) are passed explicitely as function arguments rather than being referenced directly. When testing, I'm free to pass a stub instead of the original function read-line. Is that good? Regards Thomas

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RE: [gambit-list] Newbie: define-macro question
by Logan, Patrick D 18 Aoû '05

18 Aoû '05

> In your opinion, is it appropriate to use a macro to abstract away > repetitive boiler-plate code? Or is this better done in a procedure? This is almost always a procedural abstraction rather than syntax, especially for beginners with Lisp... better to spend a lot of time with just procedural abstraction, higher-order functions, etc. Syntactical abstraction I use for controlling the order of evaluation and sometimes to put a pretty syntax around use of lambda. The begin1 is an example of control. An example of the latter use of pretty syntax... consider a scenario where you are using a resource and you want some "before" and "after" actions. The base level way to implement this is with higher-order procedures. But who wants to write (lambda () ...) all the time? So on top of this build some pretty syntax. For example... (define (call-when-ready procedure) (wait-until-ready time-out) (if (not (ready?)) (call-when-ready procedure))) Use it like this... (call-when-ready (lambda () (display "I am glad this is finally ready!") (newline) (do-something))) This is fine when someone else is generating the code for you. Normally you might want to abstract the procedure as a sequence of statements... (when-ready (display "I am glad this is finally ready!") (newline) (do-something)) And so when-ready is defined as a macro that calls call-when-ready... (define-macro (when-ready . body) `(call-when-ready (lambda () ,@body))) -Patrick

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RE: [gambit-list] Newbie: define-macro question
by Logan, Patrick D 18 Aoû '05

18 Aoû '05

> I'm rather new to Lisp & Scheme (< 3 months total exposure) but I'm > trying to make a real effort to learn the language. I'm delving into > macros and having a problem understanding the basics. > > I've defined a macro called print-line like so: > > (define-macro (print-line msg) > (list `(display ,msg) `(newline))) > > When I try to use the macro from the repl I get the error message: > > (print-line "hi") > hi > ***ERROR in (console)(a)38.1 -- Operator is not a PROCEDURE Adam gave a good explanation of this error and how to solve it. Another issue for a new Scheme programmer is *when* to use a macro. In this example there is one argument and generally you would expect that argument to be evaluated before printing. Simple procedures serve this purpose just fine, so the preferred solution would be something like... (define (print-line msg) (display msg) (newline)) This may be old news to you, and you're just using print-line as an example. But I have run into so much code over the years where programmers chose macros where procedures would be simpler. I choose to implement a macro primarily when I need to control the order of evaluation, as in a control structure like if, while, etc. As an example an old Lisp control structure is called PROG1. This structure takes a sequence of statements, evaluates each in order, and returns the result of the first statement after the last has been evaluated. This is can be expressed as a Gambit macro, but I'll call it begin1 to be more like Scheme's begin than the old Lisp's progn. > (define-macro (begin1 first-statement . remaining-statements) (let ((result-var (gensym 'first-result))) `(let ((,result-var ,first-statement)) ,@remaining-statements ,result-var))) > (begin1 1 2 3) 1 > (begin 1 2 3) 3 > (define result-var 5) > (begin 1 2 result-var) 5 > (begin result-var 1 2 3) 3 > (begin1 result-var 1 2 3) 5 > (begin1 0 (display result-var) (newline) 1 2 3) 5 0 Then the other nasty thing to watch out for when playing with control structures is the inadvertent conflict of variables introduced by the macro and those in the original statements. And so gensym is used to introduce uniquely named symbols in the macro. -Patrick

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Newbie: define-macro question
by Kevin Smith 18 Aoû '05

18 Aoû '05

I'm rather new to Lisp & Scheme (< 3 months total exposure) but I'm trying to make a real effort to learn the language. I'm delving into macros and having a problem understanding the basics. I've defined a macro called print-line like so: (define-macro (print-line msg) (list `(display ,msg) `(newline))) When I try to use the macro from the repl I get the error message: (print-line "hi") hi ***ERROR in (console)(a)38.1 -- Operator is not a PROCEDURE I do see the output I expect prior to the error so something is partially working. I'm certain that the problem is caused by my flawed understanding of how macros work. Could someone with more scheme-fu shed light on what/where the problem is? Thanks, Kevin

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RE: [gambit-list] Newbie: define-macro question
by Newcombe, Chris 18 Aoû '05

18 Aoû '05

Even though it's not a Scheme book, I'd strongly recommend reading this: http://www.paulgraham.com/onlisp.html It's one of the best books on Common Lisp style macros. Here's a recent blog entry with some useful material: http://bc.tech.coop/blog/050810.html This is a fun set of slides too: http://openmap.bbn.com/~kanderso/building/macro/ regards, Chris -----Original Message----- From: gambit-list-bounces(a)iro.umontreal.ca [mailto:gambit-list-bounces@iro.umontreal.ca] On Behalf Of Logan, Patrick D Sent: Thursday, August 18, 2005 9:09 AM To: Kevin Smith; gambit-list(a)iro.umontreal.ca Subject: RE: [gambit-list] Newbie: define-macro question > I'm rather new to Lisp & Scheme (< 3 months total exposure) but I'm > trying to make a real effort to learn the language. I'm delving into > macros and having a problem understanding the basics. > > I've defined a macro called print-line like so: > > (define-macro (print-line msg) > (list `(display ,msg) `(newline))) > > When I try to use the macro from the repl I get the error message: > > (print-line "hi") > hi > ***ERROR in (console)(a)38.1 -- Operator is not a PROCEDURE Adam gave a good explanation of this error and how to solve it. Another issue for a new Scheme programmer is *when* to use a macro. In this example there is one argument and generally you would expect that argument to be evaluated before printing. Simple procedures serve this purpose just fine, so the preferred solution would be something like... (define (print-line msg) (display msg) (newline)) This may be old news to you, and you're just using print-line as an example. But I have run into so much code over the years where programmers chose macros where procedures would be simpler. I choose to implement a macro primarily when I need to control the order of evaluation, as in a control structure like if, while, etc. As an example an old Lisp control structure is called PROG1. This structure takes a sequence of statements, evaluates each in order, and returns the result of the first statement after the last has been evaluated. This is can be expressed as a Gambit macro, but I'll call it begin1 to be more like Scheme's begin than the old Lisp's progn. > (define-macro (begin1 first-statement . remaining-statements) (let ((result-var (gensym 'first-result))) `(let ((,result-var ,first-statement)) ,@remaining-statements ,result-var))) > (begin1 1 2 3) 1 > (begin 1 2 3) 3 > (define result-var 5) > (begin 1 2 result-var) 5 > (begin result-var 1 2 3) 3 > (begin1 result-var 1 2 3) 5 > (begin1 0 (display result-var) (newline) 1 2 3) 5 0 Then the other nasty thing to watch out for when playing with control structures is the inadvertent conflict of variables introduced by the macro and those in the original statements. And so gensym is used to introduce uniquely named symbols in the macro. -Patrick _______________________________________________ Gambit-list mailing list Gambit-list(a)iro.umontreal.ca http://mailman.iro.umontreal.ca/mailman/listinfo/gambit-list

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endless loop when doing longjmp over C boundaries
by Christian 14 Aoû '05

14 Aoû '05

Hello Marc I'm writing an interface to perl, and am trying to get exception handling right. At the moment, I'm only caring about exceptions thrown in perl code (which has been called by scheme), I may be dealing with exceptions thrown in scheme code called by perl later. One way to handle perl exceptions would be to catch exceptions on every perl-from-scheme function call (using the G_EVAL flag to call_sv, like: call_sv(perlfunction,G_EVAL);), and reraise them as scheme exceptions. But I expect this to be relatively costly (several hundred cpu cycles per call), and I think placing perl exception catchers manually might also make more sense. So I'm imaging a solution like: (with-perl-exception-catcher (lambda (sv) ;; sv is the perl scalar value representing $@ (display "Got a perl exception: ") (display (sv->string sv)) #f) (lambda () (perl-call "thrower") #t)) where the perl interpreter has this perl code loaded: sub thrower { print "(thrower called)\n"; die "Hi from thrower"; } sub nothrower { print "(nothrower called)\n"; } Running the scheme program would then print "Got a perl exception: Hi from thrower at file.pl line 2" and return #f. I've got perl-call, sv->string and such already running, now I'm trying to implement with-perl-exception-catcher. Perl is using setjmp/longjmp for implementing exceptions (or, at the user's request, can throw C++ exceptions). It maintains a linked list of extended jmp_buf buffers. The exception value is stored in the perl global "$@". An exception classifyer is stored in the extended jmp buffer upon calling longjmp and copied to a user variable by the JMPENV_PUSH() macro. scheme code: (define with-perl-exception-catcher (c-lambda (scheme-object scheme-object) scheme-object "gperl__with_perl_exception_catcher")) (c-define (trampoline-0 cont) (scheme-object) scheme-object "gperl_trampoline_0" "" (cont)) (c-define (trampoline-1 cont v1) (scheme-object scheme-object) scheme-object "gperl_trampoline_1" "" (cont v1)) C code: ___SCMOBJ gperl__with_perl_exception_catcher(___SCMOBJ handler, ___SCMOBJ thunk) { if (gperl_current_interpreter) { dJMPENV; // macro from perl: declares jmp environment. int rEtV = 0; ___SCMOBJ res; JMPENV_PUSH(rEtV); // macro from perl: make the new environment // the current (and link it to the previous), // call setjmp, set rEtV to 0 at first or // to the error classifyer upon longjmp. if (rEtV == 0) { // now either throw a perl exception directly: // croak("Hello this is a perl exception"); // or call thunk through gperl-trampoline-0: // res= gperl_trampoline_0(thunk); // or call thunk through this scary code: #define ___NARGS 1 ___BEGIN_SFUN_SCMOBJ(___MLBL(___C_LBL_gperl_trampoline_0)) /* label reused :o) */ ___BEGIN_SFUN_ARG_SCMOBJ(1) #define ___ARG1 thunk ___BEGIN_SFUN_BODY ___SFUN_ARG(1,___ARG1) ___SFUN_CALL_SCMOBJ ___SFUN_SET_RESULT_SCMOBJ ___END_SFUN_BODY #undef ___ARG1 ___END_SFUN_ARG_SCMOBJ(1) ___SFUN_ERROR_SCMOBJ ___SFUN_SET_RESULT_SCMOBJ ___END_SFUN_SCMOBJ #undef ___NARGS res= ___result; fprintf(stderr,"gperl_with_perl_exception_catcher: no exception occured in perl.\n"); } JMPENV_POP; // make the previous environment the current. if (rEtV != 0) { fprintf(stderr,"gperl_with_perl_exception_catcher: an exception occured in perl!\n"); // now call handler and return it's value return gperl_trampoline_1(handler, ___FIX(1234)); //todo: replace with gperl_get_sv("$@") } else { // return thunk's value. return res; } } else { return ___FIX(-1235); //(todo: produce a scheme exception) } } Note that I've first tried to call back to the scheme lambdas using c-define'd trampoline functions. It worked just fine so far when calling the perl function "nothrower", i.e. (with-perl-exception-catcher (lambda (sv) (display "Got a perl exception\n")) (lambda () (perl-call "nothrower"))) but when calling "thrower", it is outputting "(nothrower called)", "gperl_with_perl_exception_catcher: an exception occured in perl!" and "Got a perl exception", but then is hanging in an infinite loop, which ctl-c doesn't interrupt. Call graph: scheme -> C -> scheme: thunk -> C -> perl: nothrower <- <- <- <- ok. scheme -> C -> scheme: thunk -> C -> perl: thrower <-------------------longjmp- -> scheme: handler [ somewhere here it loops: ] <- <- Then I've replaced the gperl_trampoline_0(thunk); call with the above code which is an excerpt from what the compiler produced for the former; this still works for "nothrower", but terminates the program with signal 4 (illegal instruction) for "thrower". When commenting out both the gperl_trampoline_0 and compiler-produced code and directly call the C function croak("Hello this is a perl exception"); which to the best of my knowledge acts exactly as the perl die function, there is no crash or endless loop, everything seems to work. Call graph: scheme -> C croak(); <-longjmp- -> scheme: handler <- <- So I would guess it must be the call to the thunk which makes it loop. My questions: - is it at all possible to longjmp over scheme<->C boundaries? - what is the most natural way to call back into a scheme lambda from C? (I've noticed the C closure code in os_dyn.c, is this code used in one of the above situations?) I'll be happy to give you more information or real code for testing. I've tested on Debian/x86. Thanks Christian.

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Re: [gambit-list] Re: loading modules multiple times
by Adam Langley 11 Aoû '05

11 Aoû '05

On 8/11/05, Christian <christian(a)pflanze.mine.nu> wrote: > Maybe put (load "my-load") into your main program, which has that > definition. Your modules would just use the already-loaded definition. But I don't want to have to decide which is my main program. Running any of the modules might make sense in of itself, e.g. to run tests. > Marc: > > * how can one cheaply copy a binding from one namespace to another? > I'm using (eval `(define ,fully-qualified-to ,fully-qualified-to)) > now, and that's the slowest part of the whole thing. I'm not providing an answer, but one can evaluate (##symbol-table) and (##keyword-table). They are hash tables (vectors) but I don't know what their elements are. An empty element prints as ||, but that's not readable. AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org (+44) (0)7906 332512 PGP: 9113 256A CC0F 71A6 4C84 5087 CDA5 52DF 2CB6 3D60

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Gambit-list Août 2005