Print debugging statement
From Wikipedia:
Print debugging (or tracing) is the act of watching (live or recorded) trace statements, or print statements, that indicate the flow of execution of a process. This is sometimes called printf debugging, due to the use of the printf function in C.
- Task
- Show the print debugging statements in the language.
- Demonstrate their ability to track provenance by displaying information about source code (e.g., code fragment, line and column number).
C
C doesn't have a built-in print debugging statement. However, it can be defined by users as a macro.
<lang C>#include <stdio.h>
- define DEBUG_INT(x) printf( #x " at line %d\nresult: %d\n\n", __LINE__, x)
int add(int x, int y) {
int result = x + y; DEBUG_INT(x); DEBUG_INT(y); DEBUG_INT(result); DEBUG_INT(result+1); return result;
}
int main() {
add(2, 7); return 0;
}</lang>
- Output:
x at line 7 result: 2 y at line 8 result: 7 result at line 9 result: 9 result+1 at line 10 result: 10
C++
<lang cpp>#include <iostream>
// requires support for variadic macros in this form
- define DEBUG(msg,...) fprintf(stderr, "[DEBUG %s@%d] " msg "\n", __FILE__, __LINE__, __VA_ARGS__)
// may be replace with and include of <source_location> (c++20) if it becomes part of the standard
int main() {
DEBUG("Hello world"); DEBUG("Some %d Things", 42);
return 0;
}</lang>
COBOL
Works with GnuCOBOL.
Explicit debug usually just uses DISPLAY, and usually directed at SYSERR. Compiler Directing Facility debug is supported with >>D
directive statements. GnuCOBOL also supports full on line step tracing, controlled by both compile time switch, -ftraceall
, and a truthy setting in a `COB_SET_TRACE` environment variable (so program step logging can be switched on and off without a recompile (for tracking down pesky production run mysteries)).
<lang cobol>gcobol*>
*> steptrace.cob *> Tectonics: cobc -xj -fdebugging-line -ftraceall steptrace.cob *> export COB_SET_TRACE=Y *> identification division. program-id. steptrace.
data division. working-storage section.
procedure division. steptrace-main.
display "explicit line" upon syserr
>>Ddisplay "debug line" upon syserr
display "from " FUNCTION MODULE-ID " in " FUNCTION MODULE-SOURCE goback. end program steptrace.</lang>
That is fixed form COBOL, columns 1 through 6 ignored by the preprocessor (historical format, from the days of punch card input) with column 7 being a special indicator column, star for comments, dash for continuations, and D is supported by GnuCOBOL for Debug lines. The >>D
form is a newer addition to the Compiler Directing Facility, and can float anywhere on a free format compile line.
Along with MODULE-ID
and MODULE-SOURCE
, there are intrinsics for MODULE-CALLER-ID, FUNCTION MODULE-DATE, FUNCTION MODULE-FORMATTED-DATE, FUNCTION MODULE-PATH, FUNCTION MODULE-TIME.
- Output:
prompt$ cobc -xj -fdebugging-line -ftraceall steptrace.cob explicit line debug line from steptrace in steptrace.cob prompt$ cobc -xj -ftraceall steptrace.cob explicit line from steptrace in steptrace.cob prompt$ export COB_SET_TRACE=Y prompt$ ./steptrace Source: 'steptrace.cob' Program-Id: steptrace Entry: steptrace Line: 22 Program-Id: steptrace Section: (None) Line: 22 Program-Id: steptrace Paragraph: steptrace-main Line: 22 Program-Id: steptrace Statement: DISPLAY Line: 24 explicit line Program-Id: steptrace Statement: DISPLAY Line: 28 from steptrace in steptrace.cob Program-Id: steptrace Statement: GOBACK Line: 30 Program-Id: steptrace Exit: steptrace
D
Using templates and default arguments provides options for specifying a debug function. The file and line could be included as either template arguments or function arguments. <lang d>import std.stdio;
void debugln(string file = __FILE__, size_t line = __LINE__, S...)(S args) {
write('[', file, '@', line, "] ", args, '\n');
}
void debugWrite(S...)(S args, string file = __FILE__, size_t line = __LINE__) {
write('[', file, '@', line, "] ", args, '\n');
}
void main() {
debugln(); debugln("Hello world!"); debugln("Hello", ' ', "world", '!');
debugWrite(); debugWrite("Goodbye world!"); debugWrite("Goodbye", ' ', "world", '!');
} </lang>
- Output:
[debug.d@12] [debug.d@13] Hello world! [debug.d@14] Hello world! [debug.d@16] [debug.d@17] Goodbye world! [debug.d@18] Goodbye world!
Delphi
Delphi has a "OutputDebugString" for debug, but not give lines and filename of source. <lang Delphi> program DebugApp;
{$APPTYPE CONSOLE}
uses
winapi.windows, System.sysutils;
function Add(x, y: Integer): Integer; begin
Result := x + y; OutputDebugString(PChar(format('%d + %d = %d', [x, y, result])));
end;
begin
writeln(Add(2, 7)); readln;
end.</lang>
In next we have a workaround using a idea from user3989283[[1]]. This code override the default assert function to gain access to line and file name. Obs.: In this example default action of assert is disabled, but you can enable calling in HookAssert (at end).
<lang Delphi> program DebugApp;
{$APPTYPE CONSOLE}
uses
winapi.windows, System.sysutils, System.ioutils;
var
OldAssert: TAssertErrorProc;
procedure HookAssert(const M, F: string; L: Integer; E: Pointer); var
msg: string; fFile: Text;
const
LOG_FILE = '.\Debug.log';
begin
msg := '[' + DateTimeToStr(now) + ']'; msg := msg + format(' [Line: %.4d] File: %s', [L, F]);
Assign(fFile, LOG_FILE); if FileExists(LOG_FILE) then Append(fFile) else Rewrite(fFile); Write(fFile, msg + #10); Write(fFile, M + #10); Close(fFile);
// Uncomment next line to enable the original assert function // OldAssert(M, F, L, E); end;
procedure AttachDebug; begin
OldAssert := AssertErrorProc; AssertErrorProc := HookAssert;
end;
procedure ReleaseDebug; begin
AssertErrorProc := OldAssert;
end;
function Add(x, y: Integer): Integer; begin
Result := x + y; Assert(false, (format('%d + %d = %d', [x, y, result])));
end;
begin
AttachDebug;
writeln(Add(2, 7));
ReleaseDebug;
end.</lang>
- Output:
Example log:
[26/08/2020 18:56:51] [Line: 0051] File: ...\Debug\DebugApp.dpr 2 + 7 = 9
Go
Go doesn't have a built-in print debugging statement as such. Nor does it have macros.
However, as the following example shows, it is easy enough to mimic a C-like approach by writing a short 'debug' function which can show the value of an expression and its type at the appropriate line number in the program's source code.
Note that a label for the expression (whether it's a simple variable or not) must be passed to the 'debug' function as there is no way to deduce it otherwise. <lang go>package main
import (
"fmt" "runtime"
)
type point struct {
x, y float64
}
func add(x, y int) int {
result := x + y debug("x", x) debug("y", y) debug("result", result) debug("result+1", result+1) return result
}
func debug(s string, x interface{}) {
_, _, lineNo, _ := runtime.Caller(1) fmt.Printf("%q at line %d type '%T'\nvalue: %#v\n\n", s, lineNo, x, x)
}
func main() {
add(2, 7) b := true debug("b", b) s := "Hello" debug("s", s) p := point{2, 3} debug("p", p) q := &p debug("q", q)
}</lang>
- Output:
"x" at line 14 type 'int' value: 2 "y" at line 15 type 'int' value: 7 "result" at line 16 type 'int' value: 9 "result+1" at line 17 type 'int' value: 10 "b" at line 29 type 'bool' value: true "s" at line 31 type 'string' value: "Hello" "p" at line 33 type 'main.point' value: main.point{x:2, y:3} "q" at line 35 type '*main.point' value: &main.point{x:2, y:3}
Java
<lang java>import java.util.Objects;
public class PrintDebugStatement {
/** * Takes advantage of the stack trace to determine locality for the calling function * * @param message the message to print */ private static void printDebug(String message) { Objects.requireNonNull(message);
RuntimeException exception = new RuntimeException(); StackTraceElement[] stackTrace = exception.getStackTrace(); // index 0 is this method, where the exception was created // index 1 is the calling method, at the spot where this method was invoked StackTraceElement stackTraceElement = stackTrace[1]; String fileName = stackTraceElement.getFileName(); String className = stackTraceElement.getClassName(); String methodName = stackTraceElement.getMethodName(); int lineNumber = stackTraceElement.getLineNumber();
System.out.printf("[DEBUG][%s %s.%s#%d] %s\n", fileName, className, methodName, lineNumber, message); }
private static void blah() { printDebug("Made It!"); }
public static void main(String[] args) { printDebug("Hello world."); blah();
Runnable oops = () -> printDebug("oops"); oops.run(); }
}</lang>
- Output:
[DEBUG][PrintDebugStatement.java PrintDebugStatement.main#30] Hello world. [DEBUG][PrintDebugStatement.java PrintDebugStatement.blah#26] Made It! [DEBUG][PrintDebugStatement.java PrintDebugStatement.lambda$main$0#33] oops
jq
The Go implementation supports debug and input_filename
Both the C and the Go-based implementations of jq support the `debug` and `input_filename` filters. The C implementation has additional debugging support as described below.
debug
The debug built-in is a 0-arity filter which behaves somewhat like `tee /dev/stderr` in *ix -- that is, it prints its input as a message to stderr and also passes it along to the next filter, as illustrated by this transcript: <lang sh> jq -n '"abc" | debug | length' ["DEBUG:","abc"] 3 </lang>
$__loc__
The C implementation of jq provides the variable $__loc__ for accessing the file and line number in the source code where it is located, though currently $__loc__.file only provides informative information when it is called from within a module file.
input_filename
In both the C and Go implementations, this holds the file name of the file from which data is currently being read.
input_line_number
Currently, this companion to input_filename is only supported by the C implementation of jq.
Julia
Julia has native print logging type functions, including a core Logging module. In addition, there are several additional downloadable modules, such as Memento, with extended logging functionality. Julia's builtin logging has defined macros for logging levels: @error, @warn, @info, and @debug. By default, debug level statements in code are not printed unless the logging level is set to allow debug statements to print, which can be enabled by setting the JULIA_DEBUG environment variable. <lang julia>function test()
@info "starting test()" a = [1, 2] for i in 1:4 if i > 3 @debug "debugging $a at line $(@__LINE__) of file $(@__FILE__)" else a .*= 2 end end @warn "exiting test()" println()
end
test()
ENV["JULIA_DEBUG"] = "all"
test()
</lang>
- Output:
[ Info: starting test() ┌ Warning: exiting test() └ @ Main /usr/programming/test2.jl:13 [ Info: starting test() ┌ Debug: debugging [8, 16] at line 8 of file /usr/programming/test2.jl └ @ Main /usr/programming/test2.jl:8 ┌ Warning: exiting test() └ @ Main /usr/programming/test2.jl:13
Kotlin
<lang scala>fun printDebug(message: String) {
val exception = RuntimeException() val stackTrace = exception.stackTrace val stackTraceElement = stackTrace[1] val fileName = stackTraceElement.fileName val className = stackTraceElement.className val methodName = stackTraceElement.methodName val lineNumber = stackTraceElement.lineNumber
println("[DEBUG][$fileName $className.$methodName#$lineNumber] $message")
}
fun blah() {
printDebug("Made It!")
}
fun main() {
printDebug("Hello world.") blah()
val oops = { printDebug("oops") } oops.invoke()
fun nested() { printDebug("nested") } nested()
}</lang>
- Output:
[DEBUG][PrintDebuggingStatement.kt PrintDebuggingStatementKt.main#18] Hello world. [DEBUG][PrintDebuggingStatement.kt PrintDebuggingStatementKt.blah#14] Made It! [DEBUG][PrintDebuggingStatement.kt PrintDebuggingStatementKt$main$oops$1.invoke#21] oops [DEBUG][PrintDebuggingStatement.kt PrintDebuggingStatementKt$main$1.invoke#25] nested
Mercury
Mercury has trace goals which can be used within pure code, can summon !IO
for use within the goal, can be made conditional off of compile-time or runtime flags, and which are pretty free with what they can do.
Together with data functors like $module
, $pred
, $line
, trace goals can be used for debugging print statements as in the following example. Together with the require module's utilities, trace goals can be used for assert() statements or pre/post assertions.
<lang Mercury>:- module add.
- - interface.
- - import_module io.
- - pred main(io::di, io::uo) is det.
- - implementation.
- - import_module int, io, string, list.
- - func add(int, int) = int.
A `add` B = C :-
C = A + B, trace [io(!IO), compile_time(grade(debug))] ( io.format("%s - %s(%d): %d `add` %d = %d\n", [s($grade), s($pred), i($line), i(A), i(B), i(C)], !IO) ).
main(!IO) :-
2 `add` 7 = N, io.print_line(N, !IO).</lang>
- Output:
(with a non-debug grade)
9
- Output:
(with a debug grade)
asm_fast.par.gc.debug.stseg - function `add.add'/2(13): 2 `add` 7 = 9 9
Nim
In Nim, there are many ways to implement print debugging statements.
The first one consists to insert conditional printing statements. Printing statements may write on stdout
, stderror
or any other file. In their simplest form, they can be an echo
statement or a debugEcho
statement, the latter pretending to be side effects free.
The conditional activation is done using a “when” statement. Here is an example: <lang Nim>when defined(debug):
echo "Debugging info: $1 $2 $3".format(x, y, z)</lang>
When compiling, if nothing is specified, no code is generated to display the debugging information. If we want to generate the debug statements, we have to specify -d:debug
when compiling, for instance if the file is named “example.nim”: nim c -d:debug example.nim
. Note that the name “debug” has been chosen as an example but maybe any valid Nim identifier not already used. This allows to use different flags according to what we want to debug.
If is also possible to use existing flags instead of defining new ones: <lang Nim>when not defined(release):
echo "Debugging info: ", x, " ", y, " ", z</lang>
In this case, by default (debug build), the code for the statement is generated. But if we produce a release build (-d:release
option), the debugging code will not be generated.
Note that it is possible embed a conditional print statement in a template or even to use macros to create more complicated forms of debugging statements.
A second way to insert debugging print statements consists to use “assert” statements. This is not really debugging statements, but rather sanity checks. Nim provides an “assert” statement which is deactivated if checks are off and a “doAssert” statement which cannot be deactivated. Even if the assert statement is simple, it allows to display a message which useful debugging information in case of failure.
Last, but not least, Nim provides a simple and efficient log module. Using logger objects, this module allows to write log data to the console or into a file. As expected of a logging module, it provides several levels for debugging:
<lang Nim>Level = enum
lvlAll, ## All levels active lvlDebug, ## Debug level and above are active lvlInfo, ## Info level and above are active lvlNotice, ## Notice level and above are active lvlWarn, ## Warn level and above are active lvlError, ## Error level and above are active lvlFatal, ## Fatal level and above are active lvlNone ## No levels active; nothing is logged</lang>
These levels are mapped to the following names: "DEBUG", "DEBUG", "INFO", "NOTICE", "WARN","ERROR", "FATAL", "NONE".
Changing the logging level for a logger or globally maybe done at runtime if the application has implemented some mechanism for that, or at compile time, for instance by using some flag with a value: -d:logLevel=DEBUG
.
Logging debugging data is then a simple as writing: <lang Nim>debug "Debugging info: $1 $2 $3".format(x, y, z)</lang>
Perl
Carp
is a core module, always available.
<lang perl>use Carp;
$str = 'Resistance'; carp "'$str' is futile."; print "\n";
doodle($str); print "\n";
fiddle(7);
sub doodle { my ($str) = @_; carp "'$str' is still futile." }
sub fiddle { faddle(2*shift) } sub faddle { fuddle(3*shift) } sub fuddle { ( carp("'$_[0]', interesting number.") ); }</lang>
- Output:
'Resistance' is futile. at printf_debug.pl line 11. 'Resistance' is still futile. at printf_debug.pl line 17. main::doodle("Resistance") called at printf_debug.pl line 13 '42', interesting number. at printf_debug.pl line 21. main::fuddle(42) called at printf_debug.pl line 20 main::faddle(14) called at printf_debug.pl line 19 main::fiddle(7) called at printf_debug.pl line 15
Phix
The ? statement is used as a quick shorthand to dump variable contents and expression results.
For provenance, the following was added to builtins/reflections.e (for now/not an autoinclude, 0.8.1+), and throw()
was also tweaked to convert supplied addresses, which it previously did not. It proved to be quite an easy enhancement
to the language, albeit as yet undocumented.
function _debug_info() -- use throw to convert a return address and routine number -- from the call stack into a proper line number, etc. -- (private, not called direct/from outside this file) integer rtn atom ret_addr #ilASM{ [32] mov edx,[ebp+20] -- prev_ebp mov eax,[edx+28] -- return address mov edx,[edx+20] -- prev_ebp lea edi,[ret_addr] call :%pStoreMint mov eax,[edx+8] -- calling routine no mov [rtn],eax [64] mov rdx,[rbp+40] -- prev_ebp mov rax,[rdx+56] -- return address mov rdx,[rdx+40] -- prev_ebp lea rdi,[ret_addr] call :%pStoreMint mov rax,[rdx+16] -- calling routine no mov [rtn],rax [] } try throw({1,ret_addr-1,-1,rtn,-1,-1,-1}) catch e return e end try end function -- NOTE: following five routines must all use the exact same nesting level. global function debug_info() return _debug_info() end function global function debug_line() return _debug_info()[E_LINE] end function global function debug_rtn() return _debug_info()[E_NAME] end function global function debug_file() return _debug_info()[E_FILE] end function global function debug_path() return _debug_info()[E_PATH] end function
This can be used as follows (0.8.1+)
include builtins/reflections.e ?debug_info() ?debug_line() procedure test() ?debug_info() ?debug_line() ?debug_file() printf(1,"This is line %d in file %s\n",{debug_line(),debug_file()}) end procedure test()
- Output:
{1,7544919,3,21,"-1","test.exw","C:\\Program Files (x86)\\Phix\\"} 4 {1,7545341,6,1034,"test","test.exw","C:\\Program Files (x86)\\Phix\\"} 7 "test.exw" This is line 9 in file test.exw
See the throw() documentation for more details, especially regarding the debug_info() results.
There is no routine name for the first debug_info() call, so you get "-1" in [E_NAME].
The line numbers 3, 4, 6, 7, and 9 are returned for five of the seven calls.
Python
Python's builtin logging module allows extensive customization of output format and destination. <lang python>import logging, logging.handlers
LOG_FILENAME = "logdemo.log" FORMAT_STRING = "%(levelname)s:%(asctime)s:%(name)s:%(funcName)s:line-%(lineno)d: %(message)s" LOGLEVEL = logging.DEBUG
CRITICAL 50 ERROR 40 WARNING 30 **DEFAULT INFO 20 DEBUG 10 NOTSET 0
def print_squares(number):
logger.info("In print_squares") for i in range(number): print("square of {0} is {1}".format(i , i*i)) logger.debug(f'square of {i} is {i*i}')
def print_cubes(number):
logger.info("In print_cubes") for j in range(number): print("cube of {0} is {1}".format(j, j*j*j)) logger.debug(f'cube of {j} is {j*j*j}')
if __name__ == "__main__":
logger = logging.getLogger("logdemo") logger.setLevel(LOGLEVEL) handler = logging.FileHandler(LOG_FILENAME) handler.setFormatter(logging.Formatter(FORMAT_STRING)) logger.addHandler(handler)
print_squares(10) print_cubes(10)
logger.info("All done")</lang>
- Output:
Contents of logdemo.log
INFO:2020-06-27 22:10:21,130:logdemo:print_squares:line-17: In print_squares DEBUG:2020-06-27 22:10:21,132:logdemo:print_squares:line-20: square of 0 is 0 DEBUG:2020-06-27 22:10:21,238:logdemo:print_squares:line-20: square of 1 is 1 DEBUG:2020-06-27 22:10:21,347:logdemo:print_squares:line-20: square of 2 is 4 DEBUG:2020-06-27 22:10:21,451:logdemo:print_squares:line-20: square of 3 is 9 DEBUG:2020-06-27 22:10:21,557:logdemo:print_squares:line-20: square of 4 is 16 DEBUG:2020-06-27 22:10:21,662:logdemo:print_squares:line-20: square of 5 is 25 DEBUG:2020-06-27 22:10:21,770:logdemo:print_squares:line-20: square of 6 is 36 DEBUG:2020-06-27 22:10:21,880:logdemo:print_squares:line-20: square of 7 is 49 DEBUG:2020-06-27 22:10:21,982:logdemo:print_squares:line-20: square of 8 is 64 DEBUG:2020-06-27 22:10:22,090:logdemo:print_squares:line-20: square of 9 is 81 INFO:2020-06-27 22:10:22,195:logdemo:print_cubes:line-24: In print_cubes DEBUG:2020-06-27 22:10:22,196:logdemo:print_cubes:line-27: cube of 0 is 0 DEBUG:2020-06-27 22:10:22,298:logdemo:print_cubes:line-27: cube of 1 is 1 DEBUG:2020-06-27 22:10:22,406:logdemo:print_cubes:line-27: cube of 2 is 8 DEBUG:2020-06-27 22:10:22,512:logdemo:print_cubes:line-27: cube of 3 is 27 DEBUG:2020-06-27 22:10:22,614:logdemo:print_cubes:line-27: cube of 4 is 64 DEBUG:2020-06-27 22:10:22,716:logdemo:print_cubes:line-27: cube of 5 is 125 DEBUG:2020-06-27 22:10:22,824:logdemo:print_cubes:line-27: cube of 6 is 216 DEBUG:2020-06-27 22:10:22,932:logdemo:print_cubes:line-27: cube of 7 is 343 DEBUG:2020-06-27 22:10:23,037:logdemo:print_cubes:line-27: cube of 8 is 512 DEBUG:2020-06-27 22:10:23,145:logdemo:print_cubes:line-27: cube of 9 is 729 INFO:2020-06-27 22:10:23,249:logdemo:<module>:line-41: All done
Pyret
Pyret has the spy
expression. The expression can print the value of an identifier, using the identifier itself as a label if it's not already given. It could also print the value of an arbitrary expression, but it needs an explicit label in this case.
<lang pyret>fun add(x, y):
result = x + y spy "in add": x, y, result, result-plus-one: result + 1 end result
end
add(2, 7)</lang>
- Output:
Spying "in add" (at file:///spies.arr:3:2-8:5) x: 2 y: 7 result: 9 result-plus-one: 10 9
Racket
Racket doesn't have a built-in print debugging statement. However, it can be defined by users as a macro.
<lang racket>#lang racket
(require syntax/parse/define)
(define (debug:core line col code val #:label [label #f])
;; if label exists, use it instead of the code fragment (printf "~a at line ~a column ~a\n" (or label code) line col) (printf "result: ~a\n\n" val) ;; return the value itself, so that we can wrap macro around an expression ;; without restructuring any code val)
(define-simple-macro (debug <x> option ...)
#:with line (datum->syntax this-syntax (syntax-line #'<x>)) #:with col (datum->syntax this-syntax (syntax-column #'<x>)) (debug:core line col (quote <x>) <x> option ...))
(define (add x y)
(define result (+ x y)) (debug x) (debug y) (debug (if #t (+ x y) (error 'impossible))) (debug (add1 result) #:label "result plus one") (debug result))
(add 2 7)</lang>
- Output:
x at line 20 column 9 result: 2 y at line 21 column 9 result: 7 (if #t (+ x y) (error 'impossible)) at line 22 column 9 result: 9 result plus one at line 23 column 9 result: 10 result at line 24 column 9 result: 9 9
Raku
(formerly Perl 6)
There isn't anything built-in to do this in Rakudo/Raku, though it's pretty easy to cobble something together piggybacking off of the exception system. It would probably be better to instantiate a specific "DEBUG" exception to avoid interfering with other user instantiated ad-hoc exceptions, but for a quick-and-dirty demo, this should suffice.
This example will report any payload contents passed to the exception. If you want specific information, it will need to be passed in, though some of it may be determinable through introspection. Reports the file name and line number where the "debug" call originated and unwinds the call stack to trace through the subroutine calls leading up to it. Will follow the call chain into included files and modules, though calls to the CORE setting and dispatcher are filtered out here to reduce noise.
Comments with the files line numbers are added here to make it easier to match up the debug output with the file. Typically you would be editing the file in an editor that provides line numbering so that wouldn't be necessary/helpful.
<lang perl6>my &pdb = ¨
CATCH {
when X::AdHoc { my @frames = .backtrace[*]; say .payload; for @frames { # Filter out exception handing and dispatcher frames next if .file.contains: 'SETTING' or .subname.chars < 1; printf "%sfrom file: %s,%s line: %s\n", (' ' x $++), .file, (my $s = .subname) eq '<unit>' ?? !! " sub: $s,", .line; } say ; .resume; } default {}
}
- Testing / demonstration
- helper subs #line 22
sub alpha ($a) { #line 23
pdb ('a =>', $a + 3); #line 24 pdb 'string'; #line 25 beta(7); #line 26
} #line 27 sub beta ($b) { pdb $b } #line 28 sub gamma ($c) { beta $c } #line 29 sub delta ($d) { gamma $d } #line 30
#line 31
my $a = 10; #line 32 pdb (.VAR.name, $_) with $a; #line 33 alpha($a); #line 34 delta("Δ"); #line 35 .&beta for ^3; #line 36</lang>
- Output:
($a 10) from file: debug.p6, line: 33 (a => 13) from file: debug.p6, sub: alpha, line: 24 from file: debug.p6, line: 34 string from file: debug.p6, sub: alpha, line: 25 from file: debug.p6, line: 34 7 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, sub: alpha, line: 26 from file: debug.p6, line: 34 Δ from file: debug.p6, sub: beta, line: 28 from file: debug.p6, sub: gamma, line: 29 from file: debug.p6, sub: delta, line: 30 from file: debug.p6, line: 35 0 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36 1 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36 2 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36
REXX
Since REXX is an interpretive language, it is easy to add judicious use of the say which is an easy
way to visually examine the values of any variable throughout the program's execution.
When that might not prove feasible (maybe because of copious output before the problem occurs),
using the trace instruction might be a better choice.
Some of the options for the trace instruction are to display:
- what commands have a non-zero return code
- result of clauses
- what commands are being executed
- the (name of) labels being executed
- command errors
- command failures
- commands executed that have a negative return code
- an interactive mode that pauses and lets the programmer display values of variables
One of the options that shows the detailed information is the i (for intermediate) option which is the
most informative and shows intermediate results within a REXX statement as it's being evaluated.
The first number (for the trace output) is the line number for the REXX program.
(Blank lines are not traced.)
The following output is from the Regina REXX interpreter. <lang rexx>/*REXX program to demonstrate debugging (TRACE) information while executing a program*/ /*────────────────────────────────────────────── (below) the I is for information. */ trace i parse arg maxDiv . if maxDiv== | maxDiv=="," then maxDiv= 1000 /*obtain optional argument from the CL.*/ say 'maximum random divisor is:' maxDiv /*display the max divisor being used. */ total= 0
do j=1 to 100 total= total + j/random(maxDiv) end /*j*/
say 'total=' total /*stick a fork in it, we're all done. */</lang>
- output when using the input of: 9
4 *-* parse arg maxDiv . >>> "9" >.> "" 5 *-* if maxDiv=='' | maxDiv=="," then maxDiv= 1000 /*obtain optional argument from the CL.*/ >V> "9" >L> "," >O> "0" >V> "9" >L> "" >O> "0" >U> "0" 6 *-* say 'maximum random divisor is:' maxDiv /*display the max divisor being used. */ >L> "maximum random divisor is:" >V> "9" >O> "maximum random divisor is: 9" maximum random divisor is: 9 7 *-* total= 0 >L> "0" 9 *-* do j=1 to 100 >L> "1" >L> "100" >V> "1" 10 *-* total= total + j/random(maxDiv) >V> "0" >V> "1" >V> "9" >F> "3" >O> "0.333333333" >O> "0.333333333" 11 *-* end /*j*/ 9 *-* do j=1 to 100 >V> "1" >V> "2" 10 *-* total= total + j/random(maxDiv) >V> "0.333333333" >V> "2" >V> "9" >F> "0" 10 +++ total= total + j/random(maxDiv) Error 42 running "c:\debuggin.rex", line 10: Arithmetic overflow/underflow Error 42.3: Arithmetic overflow; divisor must not be zero
Programming note: this error occurred because when the random BIF is invoked
with one argument (as is here), the
range of random numbers generated are integers from zero to the value of the argument (inclusive).
Wren
What was said in the preamble to the Go entry applies equally to Wren except that there is no way at present to detect what line number is currently being executed except by aborting the fiber and terminating the script.
What is generally done in practice is to hard-code some location indicator (such as the line number itself) at which a variable's value is being obtained. The Go example code then looks like this when translated to Wren. <lang ecmascript>import "/fmt" for Fmt
class Point {
construct new(x, y) { _x = x _y = y } x { _x } y { _y } toString { "(%(_x), %(_y))" }
}
var debug = Fn.new { |s, x, lineNo|
Fmt.print("$q at line $d type '$k'\nvalue: $s\n", s, lineNo, x, x)
}
var add = Fn.new { |x, y|
var result = x + y debug.call("x", x, 19) debug.call("y", y, 20) debug.call("result", result, 21) debug.call("result+1", result+1, 22) return result
}
add.call(2, 7) var b = true debug.call("b", b, 28) var s = "Hello" debug.call("s", s, 30) var p = Point.new(2, 3) debug.call("p", p, 32) var l = [1, "two", 3] debug.call("l", l, 34)</lang>
- Output:
"x" at line 19 type 'Num' value: 2 "y" at line 20 type 'Num' value: 7 "result" at line 21 type 'Num' value: 9 "result+1" at line 22 type 'Num' value: 10 "b" at line 28 type 'Bool' value: true "s" at line 30 type 'String' value: Hello "p" at line 32 type 'Point' value: (2, 3) "l" at line 34 type 'Num String Num' value: 1 two 3
zkl
Print debugging is similar to C. The _debug_ keyword conditionally compiles code (ie the debug code isn't compiled unless debugging is turned on). <lang zkl>fcn ds(line=__LINE__){
println("This is line %d of file %s compiled on %s" .fmt(line,__FILE__,__DATE__));
}(); _debug_{
ds(__LINE__); println("Debug level is ",__DEBUG__); vm.stackTrace().println();
}</lang>
- Output:
Run with debugging turned off: $ zkl rs This is line 39 of file rs.zkl compiled on 2019-08-28 Run with debugging turned on: Due to some brain deadness, we need to set the debug level (-d), compile the file (-c) then run it and quit out of the REPL: $ zkl -dc rs --run --quit Compiled Class(rs) (0.0 seconds, ??? lines/sec) This is line 44 of file rs.zkl compiled on 2019-08-28 This is line 49 of file rs.zkl compiled on 2019-08-28 Debug level is 1 Stack trace for VM#1 (): rs.__constructor@stackTrace addr:25 args(0) reg(0) startup.__constructor@__constructor addr:1767 args(0) reg(22) R