Print debugging statement

Print debugging statement is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

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>

1. 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

1. 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

Library: winapi.windows

Library: System.sysutils

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

Julia

Works with: 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

Works with: Python 3.6+

Library: logging

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)

Works with: Rakudo version 2019.07.1

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 {}

}

1. 1. Testing / demonstration

1. 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

Library: Wren-fmt

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