diff options
author | Neil Jerram <neil@ossau.uklinux.net> | 2006-08-11 16:21:14 +0000 |
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committer | Neil Jerram <neil@ossau.uklinux.net> | 2006-08-11 16:21:14 +0000 |
commit | 62ae95577a503e89114584ae8571b2b128d79ca5 (patch) | |
tree | 370ebdfe3574f074537f98474f3b85426e841ef8 /doc/ref/api-debug.texi | |
parent | 5af872e136b2e7c74b566b68bbab97b288464578 (diff) | |
download | guile-62ae95577a503e89114584ae8571b2b128d79ca5.tar.gz |
* scheme-using.texi (Run To Frame Exit): Improved doc for finish.
(Continue Execution): Improved doc for continue.
(Using Guile in Emacs): Lots of new docs about the Emacs
interface.
* api-debug.texi (Low Level Trap Calls): New.
(Using Traps): Removed, material incorporated into Low Level Trap
Calls.
(High Level Traps): New.
(Breakpoints): New.
* scheme-using.texi (Single Stepping): Improve doc for step and
next.
* api-debug.texi (Debug on Error): Note need to handling of errors
in C.
Diffstat (limited to 'doc/ref/api-debug.texi')
-rw-r--r-- | doc/ref/api-debug.texi | 1267 |
1 files changed, 1263 insertions, 4 deletions
diff --git a/doc/ref/api-debug.texi b/doc/ref/api-debug.texi index 42527b7ec..6868ef211 100644 --- a/doc/ref/api-debug.texi +++ b/doc/ref/api-debug.texi @@ -17,7 +17,9 @@ infrastructure that builds on top of those calls. @menu * Evaluation Model:: Evaluation and the Scheme stack. * Debug on Error:: Debugging when an error occurs. -* Using Traps:: +* Low Level Trap Calls:: +* High Level Traps:: +* Breakpoints:: @end menu @node Evaluation Model @@ -380,9 +382,48 @@ the backtrace. Invoke the Guile debugger to explore the context of the last error. @end deffn - -@node Using Traps -@subsection Using Traps +[Should also cover how to catch and debug errors from C, including +discussion of lazy/pre-unwind handlers.] + + +@node Low Level Trap Calls +@subsection Low Level Trap Calls + +@cindex Low level trap calls +@cindex Evaluator trap calls +Guile's evaluator can be configured to call three user-specified +procedures at various points in its operation: an +@dfn{apply-frame-handler} procedure, an @dfn{enter-frame-handler} +procedure, and an @dfn{exit-frame-handler} procedure. These procedures, +and the circumstances under which the evaluator calls them, are +configured by the ``evaluator trap options'' interface (@pxref{Evaluator +trap options}), and by the @code{trace} and @code{breakpoints} fields of +the ``debug options'' interface (@pxref{Debugger options}). + +It is not necessary to understand the fine details of these low level +calls, and of the options which configure them, in order to use the +class-based trap interface effectively. @code{guile-debugging} takes +care of setting these options as required for whatever set of +installed trap objects the user specifies.@footnote{And consequently, +when using the class-based trap interface, users/applications should +@emph{not} modify these options themselves, to avoid interfering with +@code{guile-debugging}'s option settings.} It is useful, though, to +have a overall idea of how the evaluator works and when these low +level calls can happen, as follows. + +@cindex Frame entry +@cindex Frame exit +On the basis of this description, we can now specify the points where +low level trap calls may occur (subject to configuration). Namely, +whenever a new frame is added to the stack, because the evaluator is +about to begin a new evaluation or to perform a new application, and +whenever a frame is being removed from the stack because the +computation that it refers to has completed and is returning its +value@footnote{If this raises the question of how expressions with +no return value are handled, the answer is that all computations in +Guile return a value. Those that appear to have no return value do so +by using the special @code{*unspecified*} value, which the Guile REPL +avoids displaying to the user.} to its caller. @deffn {Scheme Procedure} with-traps thunk @deffnx {C Function} scm_with_traps (thunk) @@ -395,6 +436,1224 @@ Return @code{#t} if @var{obj} is a debug object. @end deffn +@node High Level Traps +@subsection High Level Traps + +@cindex Traps +@cindex Evaluator trap calls +@cindex Breakpoints +@cindex Trace +@cindex Tracing +@cindex Code coverage +@cindex Profiling +The low level C code of Guile's evaluator can be configured to call +out at key points to arbitrary user-specified code. In principle this +allows Scheme code to implement any model it chooses for examining the +evaluation stack as program execution proceeds, and for suspending +execution to be resumed later. Possible applications of this feature +include breakpoints, runtime tracing, code coverage, and profiling. + +@cindex Trap classes +@cindex Trap objects +Based on these low level trap calls, the enhancements described here +provide a much higher level, object-oriented interface for the +manipulation of traps. Different kinds of trap are represented as +GOOPS classes; for example, the @code{<procedure-trap>} class +describes traps that are triggered by invocation of a specified +procedure. A particular instance of a trap class --- or @dfn{trap +object} --- describes the condition under which a single trap will be +triggered, and what will happen then; for example, an instance of +@code{<procedure-trap>} whose @code{procedure} and @code{behaviour} +slots contain @code{my-factorial} and @code{debug-trap} would be a +trap that enters the command line debugger when the +@code{my-factorial} procedure is invoked. + +The following subsubsections describe all this in greater detail, for both +the user wanting to use traps, and the developer interested in +understanding how the interface hangs together. + + +@subsubsection A Quick Note on Terminology + +@cindex Trap terminology +It feels natural to use the word ``trap'' in some form for all levels +of the structure just described, so we need to be clear on the +terminology we use to describe each particular level. The terminology +used in this subsection is as follows. + +@itemize @bullet +@item +@cindex Evaluator trap calls +@cindex Low level trap calls +``Low level trap calls'', or ``low level traps'', are the calls made +directly from the C code of the Guile evaluator. + +@item +@cindex Trap classes +``Trap classes'' are self-explanatory. + +@item +@cindex Trap objects +``Trap objects'', ``trap instances'', or just ``traps'', are instances +of a trap class, and each describe a single logical trap condition +plus behaviour as specified by the user of this interface. +@end itemize + +A good example of when it is important to be clear, is when we talk +below of behaviours that should only happen once per low level trap. +A single low level trap call will typically map onto the processing of +several trap objects, so ``once per low level trap'' is significantly +different from ``once per trap''. + + +@menu +* How to Set a Trap:: +* Specifying Trap Behaviour:: +* Trap Context:: +* Tracing Examples:: +* Tracing Configuration:: +* Tracing and (ice-9 debug):: +* Traps Installing More Traps:: +* Common Trap Options:: +* Procedure Traps:: +* Exit Traps:: +* Entry Traps:: +* Apply Traps:: +* Step Traps:: +* Source Traps:: +* Location Traps:: +* Trap Shorthands:: +* Trap Utilities:: +@end menu + + +@node How to Set a Trap +@subsubsection How to Set a Trap + +@cindex Setting traps +@cindex Installing and uninstalling traps +Setting a trap is done in two parts. First the trap is defined by +creating an instance of the appropriate trap class, with slot values +specifying the condition under which the trap will fire and the action +to take when it fires. Secondly the trap object thus created must be +@dfn{installed}. + +To make this immediately concrete, here is an example that sets a trap +to fire on the next application of the @code{facti} procedure, and to +handle the trap by entering the command line debugger. + +@lisp +(install-trap (make <procedure-trap> + #:procedure facti + #:single-shot #t + #:behaviour debug-trap)) +@end lisp + +@noindent +Briefly, the elements of this incantation are as follows. (All of +these are described more fully in the following subsubsections.) + +@itemize @bullet +@item +@code{<procedure-trap>} is the trap class for trapping on invocation +of a specific procedure. + +@item +@code{#:procedure facti} says that the specific procedure to trap on for this +trap object is @code{facti}. + +@item +@code{#:single-shot #t} says that this trap should only fire on the +@emph{next} invocation of @code{facti}, not on all future invocations +(which is the default if the @code{#:single-shot} option is not +specified). + +@item +@code{#:behaviour debug-trap} says that the trap infrastructure should +call the procedure @code{debug-trap} when this trap fires. + +@item +Finally, the @code{install-trap} call installs the trap immediately. +@end itemize + +@noindent +It is of course possible for the user to define more convenient +shorthands for setting common kinds of traps. @xref{Trap Shorthands}, +for some examples. + +The ability to install, uninstall and reinstall a trap without losing +its definition is @code{guile-debugging}'s equivalent of the +disable/enable commands provided by debuggers like GDB. + +@deffn {Generic Function} install-trap trap +Install the trap object @var{trap}, so that its behaviour will be +executed when the conditions for the trap firing are met. +@end deffn + +@deffn {Generic Function} uninstall-trap trap +Uninstall the trap object @var{trap}, so that its behaviour will +@emph{not} be executed even if the conditions for the trap firing are +met. +@end deffn + + +@node Specifying Trap Behaviour +@subsubsection Specifying Trap Behaviour + +@cindex Trap behaviour +@code{guile-debugging} provides several ``out-of-the-box'' behaviours +for common needs. All of the following can be used directly as the +value of the @code{#:behaviour} option when creating a trap object. + +@deffn {Procedure} debug-trap trap-context +Enter Guile's command line debugger to explore the stack at +@var{trap-context}, and to single-step or continue program execution +from that point. +@end deffn + +@deffn {Procedure} gds-debug-trap trap-context +Use the GDS debugging interface, which displays the stack and +corresponding source code via Emacs, to explore the stack at +@var{trap-context} and to single-step or continue program execution +from that point. +@end deffn + +@cindex Trace +@cindex Tracing +@deffn {Procedure} trace-trap trap-context +Display trace information to summarize the current @var{trap-context}. +@end deffn + +@deffn {Procedure} trace-at-exit trap-context +Install a further trap to cause the return value of the application or +evaluation just starting (as described by @var{trap-context}) to be +traced using @code{trace-trap}, when this application or evaluation +completes. The extra trap is automatically uninstalled after the +return value has been traced. +@end deffn + +@deffn {Procedure} trace-until-exit trap-context +Install a further trap so that every step that the evaluator performs +as part of the application or evaluation just starting (as described +by @var{trap-context}) is traced using @code{trace-trap}. The extra +trap is automatically uninstalled when the application or evaluation +is complete. @code{trace-until-exit} can be very useful as a first +step when all you know is that there is a bug ``somewhere in XXX or in +something that XXX calls''. +@end deffn + +@noindent +@code{debug-trap} and @code{gds-debug-trap} are provided by the modules +@code{(ice-9 debugger)} and @code{(ice-9 gds-client)} respectively, and +their behaviours are fairly self-explanatory. For more information on +the operation of the GDS interface via Emacs, see @ref{Using Guile in +Emacs}. The tracing behaviours are explained more fully below. + +@cindex Trap context +More generally, the @dfn{behaviour} specified for a trap can be any +procedure that expects to be called with one @dfn{trap context} +argument. A trivial example would be: + +@lisp +(define (report-stack-depth trap-context) + (display "Stack depth at the trap is: ") + (display (tc:depth trap-context)) + (newline)) +@end lisp + + +@node Trap Context +@subsubsection Trap Context + +The @dfn{trap context} is an object that caches information about the +low level trap call and the stack at the point of the trap, and is +passed as the only argument to all behaviour procedures. The +information in the trap context can be accessed through the procedures +beginning @code{tc:} that are exported by the @code{(ice-9 debugging +traps)} module@footnote{Plus of course any procedures that build on +these, such as the @code{trace/@dots{}} procedures exported by +@code{(ice-9 debugging trace)} (@pxref{Tracing Configuration}).}; the +most useful of these are as follows. + +@deffn {Generic Function} tc:type trap-context +Indicates the type of the low level trap by returning one of the +keywords @code{#:application}, @code{#:evaluation}, @code{#:return} or +@code{#:error}. +@end deffn + +@deffn {Generic Function} tc:return-value trap-context +When @code{tc:type} gives @code{#:return}, this provides the value +that is being returned. +@end deffn + +@deffn {Generic Function} tc:stack trap-context +Provides the stack at the point of the trap (as computed by +@code{make-stack}, but cached so that the lengthy @code{make-stack} +operation is not performed more than once for the same low level +trap). +@end deffn + +@deffn {Generic Function} tc:frame trap-context +The innermost frame of the stack at the point of the trap. +@end deffn + +@deffn {Generic Function} tc:depth trap-context +The number of frames (including tail recursive non-real frames) in the +stack at the point of the trap. +@end deffn + +@deffn {Generic Function} tc:real-depth trap-context +The number of real frames (that is, excluding the non-real frames that +describe tail recursive calls) in the stack at the point of the trap. +@end deffn + + +@node Tracing Examples +@subsubsection Tracing Examples + +The following examples show what tracing is and the kind of output that +it generates. In the first example, we define a recursive function for +reversing a list, then watch the effect of the recursive calls by +tracing each call and return value. + +@lisp +guile> (define (rev ls) + (if (null? ls) + ls + (append (rev (cdr ls)) + (list (car ls))))) +guile> (use-modules (ice-9 debugging traps) (ice-9 debugging trace)) +guile> (define t1 (make <procedure-trap> + #:procedure rev + #:behaviour (list trace-trap + trace-at-exit))) +guile> (install-trap t1) +guile> (rev '(a b c)) +| 2: [rev (a b c)] +| 3: [rev (b c)] +| 4: [rev (c)] +| 5: [rev ()] +| 5: =>() +| 4: =>(c) +| 3: =>(c b) +| 2: =>(c b a) +(c b a) +@end lisp + +@noindent +The number before the colon in this output (which follows @code{(ice-9 +debugging trace)}'s default output format) is the number of real frames +on the stack. The fact that this number increases for each recursive +call confirms that the implementation above of @code{rev} is not +tail-recursive. + +In the next example, we probe the @emph{internal} workings of +@code{rev} in more detail by using the @code{trace-until-exit} +behaviour. + +@lisp +guile> (uninstall-trap t1) +guile> (define t2 (make <procedure-trap> + #:procedure rev + #:behaviour (list trace-trap + trace-until-exit))) +guile> (install-trap t2) +guile> (rev '(a b)) +| 2: [rev (a b)] +| 2: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls)))) +| 3: (null? ls) +| 3: [null? (a b)] +| 3: =>#f +| 2: (append (rev (cdr ls)) (list (car ls))) +| 3: (rev (cdr ls)) +| 4: (cdr ls) +| 4: [cdr (a b)] +| 4: =>(b) +| 3: [rev (b)] +| 3: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls)))) +| 4: (null? ls) +| 4: [null? (b)] +| 4: =>#f +| 3: (append (rev (cdr ls)) (list (car ls))) +| 4: (rev (cdr ls)) +| 5: (cdr ls) +| 5: [cdr (b)] +| 5: =>() +| 4: [rev ()] +| 4: (if (null? ls) ls (append (rev (cdr ls)) (list (car ls)))) +| 5: (null? ls) +| 5: [null? ()] +| 5: =>#t +| 4: (list (car ls)) +| 5: (car ls) +| 5: [car (b)] +| 5: =>b +| 4: [list b] +| 4: =>(b) +| 3: [append () (b)] +| 3: =>(b) +| 3: (list (car ls)) +| 4: (car ls) +| 4: [car (a b)] +| 4: =>a +| 3: [list a] +| 3: =>(a) +| 2: [append (b) (a)] +| 2: =>(b a) +(b a) +@end lisp + +@noindent +The output in this case shows every step that the evaluator performs +in evaluating @code{(rev '(a b))}. + + +@node Tracing Configuration +@subsubsection Tracing Configuration + +The detail of what gets printed in each trace line, and the port to +which tracing is written, can be configured by the procedures +@code{set-trace-layout} and @code{trace-port}, both exported by the +@code{(ice-9 debugging trace)} module. + +@deffn {Procedure with Setter} trace-port +Get or set the port to which tracing is printed. The default is the +value of @code{(current-output-port)} when the @code{(ice-9 debugging +trace)} module is first loaded. +@end deffn + +@deffn {Procedure} set-trace-layout format-string . arg-procs +Layout each trace line using @var{format-string} and @var{arg-procs}. +For each trace line, the list of values to be printed is obtained by +calling all the @var{arg-procs}, passing the trap context as the only +parameter to each one. This list of values is then formatted using +the specified @var{format-string}. +@end deffn + +@noindent +The @code{(ice-9 debugging trace)} module exports a set of arg-proc +procedures to cover most common needs, with names beginning +@code{trace/}. These are all implemented on top of the @code{tc:} trap +context accessor procedures documented in @ref{Trap Context}, and if any +trace output not provided by the following is needed, it should be +possible to implement based on a combination of the @code{tc:} +procedures. + +@deffn {Procedure} trace/pid trap-context +An arg-proc that returns the current process ID. +@end deffn + +@deffn {Procedure} trace/stack-id trap-context +An arg-proc that returns the stack ID of the stack in which the +current trap occurred. +@end deffn + +@deffn {Procedure} trace/stack-depth trap-context +An arg-proc that returns the length (including non-real frames) of the +stack at the point of the current trap. +@end deffn + +@deffn {Procedure} trace/stack-real-depth trap-context +An arg-proc that returns the length excluding non-real frames of the +stack at the point of the current trap. +@end deffn + +@deffn {Procedure} trace/stack trap-context +An arg-proc that returns a string summarizing stack information. This +string includes the stack ID, real depth, and count of additional +non-real frames, with the format @code{"~a:~a+~a"}. +@end deffn + +@deffn {Procedure} trace/source-file-name trap-context +An arg-proc that returns the name of the source file for the innermost +stack frame, or an empty string if source is not available for the +innermost frame. +@end deffn + +@deffn {Procedure} trace/source-line trap-context +An arg-proc that returns the line number of the source code for the +innermost stack frame, or zero if source is not available for the +innermost frame. +@end deffn + +@deffn {Procedure} trace/source-column trap-context +An arg-proc that returns the column number of the start of the source +code for the innermost stack frame, or zero if source is not available +for the innermost frame. +@end deffn + +@deffn {Procedure} trace/source trap-context +An arg-proc that returns the source location for the innermost stack +frame. This is a string composed of file name, line and column number +with the format @code{"~a:~a:~a"}, or an empty string if source is not +available for the innermost frame. +@end deffn + +@deffn {Procedure} trace/type trap-context +An arg-proc that returns a three letter abbreviation indicating the +type of the current trap: @code{"APP"} for an application frame, +@code{"EVA"} for an evaluation, @code{"RET"} for an exit trap, or +@code{"ERR"} for an error (pseudo-)trap. +@end deffn + +@deffn {Procedure} trace/real? trap-context +An arg-proc that returns @code{" "} if the innermost stack frame is a +real frame, or @code{"t"} if it is not. +@end deffn + +@deffn {Procedure} trace/info trap-context +An arg-proc that returns a string describing the expression being +evaluated, application being performed, or return value, according to +the current trap type. +@end deffn + +@noindent +@code{trace/stack-depth} and @code{trace/stack-real-depth} are identical +to the trap context methods @code{tc:depth} and @code{tc:real-depth} +described before (@pxref{Trap Context}), but renamed here for +convenience. + +The default trace layout, as exhibited by the examples of the previous +subsubsubsection, is set by this line of code from the @code{(ice-9 debugging +traps)} module: + +@lisp +(set-trace-layout "|~3@@a: ~a\n" trace/stack-real-depth trace/info) +@end lisp + +@noindent +If we rerun the first of those examples, but with trace layout +configured to show source location and trap type in addition, the +output looks like this: + +@lisp +guile> (set-trace-layout "| ~25a ~3@@a: ~a ~a\n" + trace/source + trace/stack-real-depth + trace/type + trace/info) +guile> (rev '(a b c)) +| standard input:29:0 2: APP [rev (a b c)] +| standard input:4:21 3: APP [rev (b c)] +| standard input:4:21 4: APP [rev (c)] +| standard input:4:21 5: APP [rev ()] +| standard input:2:9 5: RET =>() +| standard input:4:13 4: RET =>(c) +| standard input:4:13 3: RET =>(c b) +| standard input:4:13 2: RET =>(c b a) +(c b a) +@end lisp + + +@node Tracing and (ice-9 debug) +@subsubsection Tracing and (ice-9 debug) + +The @code{(ice-9 debug)} module of the core Guile distribution +provides a tracing facility that is roughly similar to that described +here, but there are important differences. + +@itemize @bullet +@item +The @code{(ice-9 debug)} trace gives a nice pictorial view of changes +in stack depth, by using indentation like this: + +@lisp +[fact1 4] +| [fact1 3] +| | [fact1 2] +| | | [fact1 1] +| | | | [fact1 0] +| | | | 1 +| | | 1 +| | 2 +| 6 +24 +@end lisp + +However its output can @emph{only} show the information seen here, +which corresponds to @code{guile-debugging}'s @code{trace/info} +procedure; it cannot be configured to show other pieces of information +about the trap context in the way that @code{guile-debugging}'s trace +feature can. + +@item +The @code{(ice-9 debug)} trace only allows the tracing of procedure +applications and their return values, whereas @code{guile-debugging}'s +trace allows any kind of trap to be traced. + +It's interesting to note that @code{(ice-9 debug)}'s restriction here, +which might initially appear to be just a straightforward consequence +of its implementation, is also somewhat dictated by its pictorial +display. The use of indentation in the output relies on hooking into +the low level trap calls in such a way that the trapped application +entries and exits exactly balance each other. +@code{guile-debugging}'s more general traps interface allows traps to +be installed such that entry and exit traps don't necessarily balance, +which means that, in general, indentation diagrams like the one above +don't work. +@end itemize + +It isn't currently possible to use both @code{(ice-9 debug)} trace and +@code{guile-debugging} in the same Guile session, because their settings +of the low level trap options conflict with each other. (It should be +possible to fix this, by modifying @code{(ice-9 debug)} to use +@code{guile-debugging}'s trap installation interface, but only if and +when @code{guile-debugging} is integrated into the core Guile +distribution.) + + +@node Traps Installing More Traps +@subsubsection Traps Installing More Traps + +Sometimes it is desirable for the behaviour at one trap to install +further traps. In other words, the behaviour is something like +``Don't do much right now, but set things up to stop after two or +three more steps'', or ``@dots{} when this frame completes''. This is +absolutely fine. For example, it is easy to code a generic ``do +so-and-so when the current frame exits'' procedure, which can be used +wherever a trap context is available, as follows. + +@lisp +(define (at-exit trap-context behaviour) + (install-trap (make <exit-trap> + #:depth (tc:depth trap-context) + #:single-shot #t + #:behaviour behaviour))) +@end lisp + +To continue and pin down the example, this could then be used as part +of a behaviour whose purpose was to measure the accumulated time spent +in and below a specified procedure. + +@lisp +(define calls 0) +(define total 0) + +(define accumulate-time + (lambda (trap-context) + (set! calls (+ calls 1)) + (let ((entry (current-time))) + (at-exit trap-context + (lambda (ignored) + (set! total + (+ total (- (current-time) + entry)))))))) + +(install-trap (make <procedure-trap> + #:procedure my-proc + #:behaviour accumulate-time)) +@end lisp + + +@node Common Trap Options +@subsubsection Common Trap Options + +When creating any kind of trap object, settings for the trap being +created are specified as options on the @code{make} call using syntax +like this: + +@lisp +(make <@var{trap-class}> + #:@var{option-keyword} @var{setting} + @dots{}) +@end lisp + +The following common options are provided by the base class +@code{<trap>}, and so can be specified for any kind of trap. + +@deffn {Class} <trap> +Base class for trap objects. +@end deffn + +@deffn {Trap Option} #:condition thunk +If not @code{#f}, this is a thunk which is called when the trap fires, +to determine whether trap processing should proceed any further. If +the thunk returns @code{#f}, the trap is basically suppressed. +Otherwise processing continues normally. (Default value @code{#f}.) +@end deffn + +@deffn {Trap Option} #:skip-count count +A count of valid (after @code{#:condition} processing) firings of this +trap to skip. (Default value 0.) +@end deffn + +@deffn {Trap Option} #:single-shot boolean +If not @code{#f}, this indicates that the trap should be automatically +uninstalled after it has successfully fired (after @code{#:condition} +and @code{#:skip-count} processing) for the first time. (Default +value @code{#f}.) +@end deffn + +@deffn {Trap Option} #:behaviour behaviour-proc +A trap behaviour procedure --- as discussed in the preceding subsubsection +--- or a list of such procedures, in which case each procedure is +called in turn when the trap fires. (Default value @code{'()}.) +@end deffn + +@deffn {Trap Option} #:repeat-identical-behaviour boolean +Normally, if multiple trap objects are triggered by the same low level +trap, and they request the same behaviour, it's only actually useful +to do that behaviour once (per low level trap); so by default multiple +requests for the same behaviour are coalesced. If this option is set +other than @code{#f}, the contents of the @code{#:behaviour} option +are uniquified so that they avoid being coalesced in this way. +(Default value @code{#f}.) +@end deffn + + +@node Procedure Traps +@subsubsection Procedure Traps + +The @code{<procedure-trap>} class implements traps that are triggered +upon application of a specified procedure. Instances of this class +should use the @code{#:procedure} option to specify the procedure to +trap on. + +@deffn {Class} <procedure-trap> +Class for traps triggered by application of a specified procedure. +@end deffn + +@deffn {Trap Option} #:procedure procedure +Specifies the procedure to trap on. +@end deffn + +@noindent +Example: + +@lisp +(install-trap (make <procedure-trap> + #:procedure my-proc + #:behaviour (list trace-trap + trace-until-exit))) +@end lisp + + +@node Exit Traps +@subsubsection Exit Traps + +The @code{<exit-trap>} class implements traps that are triggered upon +stack frame exit past a specified stack depth. Instances of this +class should use the @code{#:depth} option to specify the target stack +depth. + +@deffn {Class} <exit-trap> +Class for traps triggered by exit past a specified stack depth. +@end deffn + +@deffn {Trap Option} #:depth depth +Specifies the reference depth for the trap. +@end deffn + +@noindent +Example: + +@lisp +(define (trace-at-exit trap-context) + (install-trap (make <exit-trap> + #:depth (tc:depth trap-context) + #:single-shot #t + #:behaviour trace-trap))) +@end lisp + +@noindent +(This is the actual definition of the @code{trace-at-exit} behaviour.) + + +@node Entry Traps +@subsubsection Entry Traps + +The @code{<entry-trap>} class implements traps that are triggered upon +any stack frame entry. No further parameters are needed to specify an +instance of this class, so there are no class-specific trap options. +Note that it remains possible to use the common trap options +(@pxref{Common Trap Options}), for example to set a trap for the +@var{n}th next frame entry. + +@deffn {Class} <entry-trap> +Class for traps triggered by any stack frame entry. +@end deffn + +@noindent +Example: + +@lisp +(install-trap (make <entry-trap> + #:skip-count 5 + #:behaviour gds-debug-trap)) +@end lisp + + +@node Apply Traps +@subsubsection Apply Traps + +The @code{<apply-trap>} class implements traps that are triggered upon +any procedure application. No further parameters are needed to +specify an instance of this class, so there are no class-specific trap +options. Note that it remains possible to use the common trap options +(@pxref{Common Trap Options}), for example to set a trap for the next +application where some condition is true. + +@deffn {Class} <apply-trap> +Class for traps triggered by any procedure application. +@end deffn + +@noindent +Example: + +@lisp +(install-trap (make <apply-trap> + #:condition my-condition + #:behaviour gds-debug-trap)) +@end lisp + + +@node Step Traps +@subsubsection Step Traps + +The @code{<step-trap>} class implements traps that do single-stepping +through a program's execution. They come in two flavours, with and +without a specified file name. If a file name is specified, the trap +is triggered by the next evaluation, application or frame exit +pertaining to source code from the specified file. If a file name is +not specified, the trap is triggered by the next evaluation, +application or frame exit from any file (or for code whose source +location was not recorded), in other words by the next evaluator step +of any kind. + +The design goal of the @code{<step-trap>} class is to match what a +user would intuitively think of as single-stepping through their code, +either through code in general (roughly corresponding to GDB's +@code{step} command, for example), or through code from a particular +source file (roughly corresponding to GDB's @code{next}). Therefore +if you are using @code{guile-debugging} to single-step through code +and finding its behaviour counter-intuitive, please let me know so +that I can improve it. + +The implementation and options of the @code{<step-trap>} class are +complicated by the fact that it is unreliable to determine whether a +low level frame exit trap is applicable to a specified file by +examining the details of the reported frame. This is a consequence of +tail recursion, which has the effect that many frames can be removed +from the stack at once, with only the outermost frame being reported +by the low level trap call. The effects of this on the +@code{<step-trap>} class are such as to require the introduction of +the strange-looking @code{#:exit-depth} option, for the following +reasons. + +@itemize @bullet +@item +When stopped at the start of an application or evaluation frame, and +it is desired to continue execution until the next ``step'' in the same +source file, that next step could be the start of a nested application +or evaluation frame, or --- if the procedure definition is in a +different file, for example --- it could be the exit from the current +frame. + +@item +Because of the effects of tail recursion noted above, the current +frame exit possibility must be expressed as frame exit past a +specified stack depth. When an instance of the @code{<step-trap>} +class is installed from the context of an application or evaluation +frame entry, the @code{#:exit-depth} option should be used to specify +this stack depth. + +@item +When stopped at a frame exit, on the other hand, we know that the next +step must be an application or evaluation frame entry. In this +context the @code{#:exit-depth} option is not needed and should be +omitted or set to @code{#f}. +@end itemize + +@noindent +When a step trap is installed without @code{#:single-shot #t}, such +that it keeps firing, the @code{<step-trap>} code automatically +updates its idea of the @code{#:exit-depth} setting each time, so that +the trap always fires correctly for the following step. + +@deffn {Class} <step-trap> +Class for single-stepping traps. +@end deffn + +@deffn {Trap Option} #:file-name name +If not @code{#f}, this is a string containing the name of a source +file, and restricts the step trap to evaluation steps within that +source file. (Default value @code{#f}.) +@end deffn + +@deffn {Trap Option} #:exit-depth depth +If not @code{#f}, this is a positive integer implying that the next +step may be frame exit past the stack depth @var{depth}. See the +discussion above for more details. (Default value @code{#f}.) +@end deffn + +@noindent +Example: + +@lisp +(install-trap (make <step-trap> + #:file-name (frame-file-name + (stack-ref stack index)) + #:exit-depth (- (stack-length stack) + (stack-ref stack index)) + #:single-shot #t + #:behaviour debug-trap)) +@end lisp + + +@node Source Traps +@subsubsection Source Traps + +The @code{<source-trap>} class implements traps that are attached to a +precise source code expression, as read by the reader, and which fire +each time that that expression is evaluated. These traps use a low +level Guile feature which can mark individual expressions for +trapping, and are relatively efficient. But it can be tricky to get +at the source expression in the first place, and these traps are +liable to become irrelevant if the procedure containing the expression +is reevaluated; these issues are discussed further below. + +@deffn {Class} <source-trap> +Class for traps triggered by evaluation of a specific Scheme +expression. +@end deffn + +@deffn {Trap Option} #:expression expr +Specifies the Scheme expression to trap on. +@end deffn + +@noindent +Example: + +@lisp +(display "Enter an expression: ") +(let ((x (read))) + (install-trap (make <source-trap> + #:expression x + #:behaviour (list trace-trap + trace-at-exit))) + (primitive-eval x)) +@print{} +Enter an expression: (+ 1 2 3 4 5 6) +| 3: (+ 1 2 3 4 5 6) +| 3: =>21 +21 +@end lisp + +The key point here is that the expression specified by the +@code{#:expression} option must be @emph{exactly} (i.e. @code{eq?} to) +what is going to be evaluated later. It doesn't work, for example, to +say @code{#:expression '(+ x 3)}, with the expectation that the trap +will fire whenever evaluating any expression @code{(+ x 3)}. + +The @code{trap-here} macro can be used in source code to create and +install a source trap correctly. Take for example the factorial +function defined in the @code{(ice-9 debugging example-fns)} module: + +@lisp +(define (fact1 n) + (if (= n 0) + 1 + (* n (fact1 (- n 1))))) +@end lisp + +@noindent +To set a source trap on a particular expression --- let's say the +expression @code{(= n 0)} --- edit the code so that the expression is +enclosed in a @code{trap-here} macro call like this: + +@lisp +(define (fact1 n) + (if (trap-here (= n 0) #:behaviour debug-trap) + 1 + (* n (fact1 (- n 1))))) +@end lisp + +@deffn {Macro} trap-here expression . trap-options +Install a source trap with options @var{trap-options} on +@var{expression}, then return with the whole call transformed to +@code{(begin @var{expression})}. +@end deffn + +Note that if the @code{trap-here} incantation is removed, and +@code{fact1} then redefined by reloading its source file, the effect +of the source trap is lost, because the text ``(= n 0)'' is read again +from scratch and becomes a new expression @code{(= n 0)} which does +not have the ``trap here'' mark on it. + +If the semantics and setting of source traps seem unwieldy, location +traps may meet your need more closely; these are described in the +following subsubsection. + + +@node Location Traps +@subsubsection Location Traps + +The @code{<location-trap>} class implements traps that are triggered +by evaluation of code at a specific source location or within a +specified range of source locations. When compared with source traps, +they are easier to set, and do not become irrelevant when the relevant +code is reloaded; but unfortunately they are considerably less +efficient, as they require running some ``are we in the right place +for a trap'' code on every low level frame entry trap call. + +@deffn {Class} <location-trap> +Class for traps triggered by evaluation of code at a specific source +location or in a specified range of source locations. +@end deffn + +@deffn {Trap Option} #:file-regexp regexp +A regular expression specifying the filenames that will match this +trap. This option must be specified when creating a location trap. +@end deffn + +@deffn {Trap Option} #:line line-spec +If specified, @var{line-spec} describes either a single line, in which +case it is a single integer, or a range of lines, in which case it is +a pair of the form @code{(@var{min-line} . @var{max-line})}. All line +numbers are 0-based, and the range form is inclusive-inclusive. If +@code{#f} or not specified, the trap is not restricted by line number. +(Default value @code{#f}.) +@end deffn + +@deffn {Trap Option} #:column column-spec +If specified, @var{column-spec} describes either a single column, in +which case it is a single integer, or a range of columns, in which +case it is a pair of the form @code{(@var{min-column} +. @var{max-column})}. All column numbers are 0-based, and the range +form is inclusive-inclusive. If @code{#f} or not specified, the trap +is not restricted by column number. (Default value @code{#f}.) +@end deffn + +@noindent +Example: + +@lisp +(install-trap (make <location-trap> + #:file-regexp "example-fns.scm" + #:line '(11 . 13) + #:behaviour gds-debug-trap)) +@end lisp + + +@node Trap Shorthands +@subsubsection Trap Shorthands + +If the code described in the preceding subsubsections for creating and +manipulating traps seems a little long-winded, it is of course +possible to define more convenient shorthand forms for typical usage +patterns. For example, my own @file{.guile} file contains the +following definitions for setting breakpoints and for tracing. + +@lisp +(define (break! proc) + (install-trap (make <procedure-trap> + #:procedure proc + #:behaviour gds-debug-trap))) + +(define (trace! proc) + (install-trap (make <procedure-trap> + #:procedure proc + #:behaviour (list trace-trap + trace-at-exit)))) + +(define (trace-subtree! proc) + (install-trap (make <procedure-trap> + #:procedure proc + #:behaviour (list trace-trap + trace-until-exit)))) +@end lisp + +Definitions like these are not provided out-of-the-box by +@code{guile-debugging}, because different users will have different +ideas about what their default debugger should be, or, for example, +which of the common trap options (@pxref{Common Trap Options}) it +might be useful to expose through such shorthand procedures. + + +@node Trap Utilities +@subsubsection Trap Utilities + +@code{list-traps} can be used to print a description of all known trap +objects. This uses a weak value hash table, keyed by a trap index +number. Each trap object has its index number assigned, and is added +to the hash table, when it is created by a @code{make @var{trap-class} +@dots{}} call. When a trap object is GC'd, it is automatically +removed from the hash table, and so no longer appears in the output +from @code{list-traps}. + +@deffn {Variable} all-traps +Weak value hash table containing all known trap objects. +@end deffn + +@deffn {Procedure} list-traps +Print a description of all known trap objects. +@end deffn + +The following example shows a single trap that traces applications of +the procedure @code{facti}. + +@lisp +guile> (list-traps) +#<<procedure-trap> 100d2e30> is an instance of class <procedure-trap> +Slots are: + number = 1 + installed = #t + condition = #f + skip-count = 0 + single-shot = #f + behaviour = (#<procedure trace-trap (trap-context)>) + repeat-identical-behaviour = #f + procedure = #<procedure facti (n a)> +@end lisp + +When @code{all-traps} or @code{list-traps} reveals a trap that you +want to modify but no longer have a reference to, you can retrieve the +trap object by calling @code{get-trap} with the trap's number. For +example, here's how you could change the behaviour of the trap listed +just above. + +@lisp +(slot-set! (get-trap 1) 'behaviour (list debug-trap)) +@end lisp + +@deffn {Procedure} get-trap number +Return the trap object with the specified @var{number}, or @code{#f} +if there isn't one. +@end deffn + + +@node Breakpoints +@subsection Breakpoints + +While they are an important piece of infrastructure, and directly +usable in some scenarios, traps are still too low level to meet some +of the requirements of interactive development. + +For example, in my experience a common scenario is that a newly +written procedure is not working properly, and so you'd like to be +able to step or trace through its code to find out why. Ideally this +should be possible from the IDE and without having to modify the +source code. There are two problems with using traps directly in this +scenario. + +@enumerate +@item +They are too detailed: constructing and installing a trap requires you +to say what kind of trap you want and to specify fairly low level +options for it, whereas what you really want is just to say ``break +here using the most efficient means possible.'' + +@item +The most efficient kinds of trap --- that is, @code{<procedure-trap>} +and @code{<source-trap>} --- can only be specified and installed +@emph{after} the code that they refer to has been loaded. This is an +inconvenient detail for the user to deal with, and in some +applications it might be very difficult to insert an instruction to +install the required trap in between when the code is loaded and when +the procedure concerned is first called. It would be better to be +able to tell Guile about the requirement upfront, and for it to deal +with installing the trap when possible. +@end enumerate + +We solve these problems by introducing breakpoints. A breakpoint is +something which says ``I want to break at location X, or in procedure +P --- just make it happen'', and can be set regardless of whether the +relevant code has already been loaded. Breakpoints use traps to do +their work, but that is a detail that the user will usually not have +to care about. + +Breakpoints are provided by a combination of Scheme code in the client +program, and facilities for setting and managing breakpoints in the +GDS front end. On the Scheme side the entry points are as follows. + +@deffn {Getter with Setter} default-breakpoint-behaviour +A ``getter with setter'' procedure that can be used to get or set the +default behaviour for new breakpoints. When a new default behaviour +is set, by calling + +@lisp +(set! (default-breakpoint-behaviour) @var{new-behaviour}) +@end lisp + +@noindent +the new behaviour applies to all following @code{break-in} and +@code{break-at} calls, but does not affect breakpoints which have +already been set. @var{new-behaviour} should be a behaviour procedure +with the signature + +@lisp +(lambda (trap-context) @dots{}) +@end lisp + +@noindent +as described in @ref{Specifying Trap Behaviour}. +@end deffn + +@deffn {Procedure} break-in procedure-name [module-or-file-name] [options] +Set a breakpoint on entry to the procedure named @var{procedure-name}, +which should be a symbol. @var{module-or-file-name}, if present, is +the name of the module (a list of symbols) or file (a string) which +includes the target procedure. If @var{module-or-file-name} is +absent, the target procedure is assumed to be in the current module. + +The available options are any of the common trap options +(@pxref{Common Trap Options}), and are used when creating the +breakpoint's underlying traps. The default breakpoint behaviour +(given earlier to @code{default-breakpoint-behaviour}) is only used if +these options do not include @code{#:behaviour @var{behaviour}}. +@end deffn + +@deffn {Procedure} break-at file-name line column [options] +Set a breakpoint on the expression in file @var{file-name} whose +opening parenthesis is on line @var{line} at column @var{column}. +@var{line} and @var{column} both count from 0 (not from 1). + +The available options are any of the common trap options +(@pxref{Common Trap Options}), and are used when creating the +breakpoint's underlying traps. The default breakpoint behaviour +(given earlier to @code{default-breakpoint-behaviour}) is only used if +these options do not include @code{#:behaviour @var{behaviour}}. +@end deffn + +@deffn {Procedure} set-gds-breakpoints +Ask the GDS front end for a list of breakpoints to set, and set these +using @code{break-in} and @code{break-at} as appropriate. +@end deffn + +@code{default-breakpoint-behaviour}, @code{break-in} and +@code{break-at} allow an application's startup code to specify any +breakpoints that it needs inline in that code. For example, to trace +calls and arguments to a group of procedures to handle HTTP requests, +one might write something like this: + +@lisp +(use-modules (ice-9 debugging breakpoints) + (ice-9 debugging trace)) + +(set! (default-breakpoint-behaviour) trace-trap) + +(break-in 'handle-http-request '(web http)) +(break-in 'read-http-request '(web http)) +(break-in 'decode-form-data '(web http)) +(break-in 'send-http-response '(web http)) +@end lisp + +@code{set-gds-breakpoints} can be used as well as or instead of the +above, and is intended to be the most practical option if you are +using GDS. The idea is that you only need to add this one call +somewhere in your application's startup code, like this: + +@lisp +(use-modules (ice-9 gds-client)) +(set-gds-breakpoints) +@end lisp + +@noindent +and then all the details of the breakpoints that you want to set can +be managed through GDS. For the details of GDS's breakpoints +interface, see @ref{Setting and Managing Breakpoints}. + + @c Local Variables: @c TeX-master: "guile.texi" @c End: |