## NAME Dawa -- A runtime debugger for Raku programs ## EXAMPLES Use from the command line with the `dawa` command: dawa-2 Use from within a program with the `stop` statement: dawa-1 ## SYNOPSIS In example.raku: use Dawa; my $x = 100; $x = $x + 1; stop; $x = $x + 10000; $x++; $x++; $x++; Then: ∙ Stopping thread Thread #1 (Initial thread) --- current stack --- in block at eg/example.raku line 5 -- current location -- 1 │ │ use Dawa; 2 │ │ 3 │ │ my $x = 100; 4 │ │ $x = $x + 1; 5 │ [1]▶ │ stop; 6 │ │ $x = $x + 10000; 7 │ │ $x++; 8 │ │ $x++; 9 │ │ $x++; 10 │ │ Type h for help dawa [1]> $x 101 dawa [1]> n 2 │ 3 │ my $x = 100; 4 │ $x = $x + 1; 5 │ stop; 6 ▷ │ $x = $x + 10000; 7 │ $x++; 8 │ $x++; 9 │ $x++; dawa [1]> $x 10101 dawa [1]> In the example above `[1]▶` indicates that this is the most recent statement that was executed on thread 1. After typing "n", the `▷` indicates the statement that was just executed. Continue typing "n", and statements will continue to be executed. A command line program, `dawa`, will run a program and stop after running a statement on the first line. ## DESCRIPTION Dawa is a run-time debugger for Raku programs. It exports one subroutine: `stop`, which will pause execution of the current thread of the program, and allow for introspecting the stack, and stepping through subsequent statements. It also supports debugging of multi-threaded programs. Importing `Dawa` adds extra ops to the AST that is generated during compilation. Specifically, it adds at most one extra node per line. There is a significant perforance penalty for this, as well as a risk that your program will break. Patches welcome! The functionality of `dawa` is inspired by Ruby's [pry](https://github.com/pry/pry) and Python's [pdb](https://docs.python.org/3/library/pdb.html). ## USAGE After `stop` is reached, a repl is started, which has a few commands. Type `h` to see them. Currently, these are the commands: break (b) : [N [filename] ] add a breakpoint at line N [in filename] continue (c, ^D) : continue execution of this thread defer (d) : [n] Defer to thread [n], or the next waiting one eval (e) : [id] evaluate code in the current context [or in thread #id] help (h) : this help ls (l) : [-a] [id] show [all] lexical variables in the current scope [or in thread #id] next (n) : run the next statement in any thread quit (q) : terminate the program (exit) step (s) : execute the next statement in the same thread threads (t) : [id] show threads being tracked [or just thread #id] where (w) : show a stack trace and the current location in the code Anything else is treated as a Raku statement: it will be evaluated, the result will be shown. ### Breakpoints Breakpoints can be set with `b`, for example: dawa [1]> b 13 Added breakpoint at line 13 in eg/debug.raku dawa [1]> w 10 │ │ 11 │ [1]▶ │ say "three"; 12 │ │ say "four"; 13 │ ■ │ $x = $x + 11; 14 │ │ say "five"; 15 │ │ say "x is $x"; 16 │ │ say "bye"; 17 │ │ As shown above, breakpoints are indicated using `■` and are not thread-specific. The triangle (▶) is the line of code that was just executed. The `[1]` indicates that this is thread 1. The `[1]` in the prompt indicates that statements will currently be evaluated in the context of thread 1 by default. ## Details of "next" In Raku, there can be multiple statements per line, and multiple "statements" within a statement, which makes it tricky to debug a program by stepping through "statements" or stepping through "lines". Dawa has a number of heuristics for when to stop, but the idea is to stop at most once per line and at most once per statement. Typing "n" may sometimes stay within the same block of code, if a statement containing other statements spans several lines. The display will indicate the block of code containing the statement that was just executed. ## Multiple Threads If several threads are stopped at once, a lock is used in order to only have one repl at a time. Threads wait for this lock. After either continuing or going on to the next statement, another thread that is waiting for the lock may potentially become active in the repl. i.e. "next" advances to the next statement in any thread. To stay in the same thread, use "step". ∙ Stopping thread Thread #1 (Initial thread) --- current stack --- in block at example.raku line 11 -- current location -- 3 │ │ my $i = 1; 4 │ │ 5 │ │ start loop { 6 │ │ $i++; 7 │ [7] │ $i++; 8 │ │ } 9 │ │ 10 │ [1]▶ │ stop; 11 │ │ say 'bye!'; 12 │ │ Type h for help dawa [1]> Note that in this situation thread 7 continues to run even while the repl is active. To also stop thread 7 while debugging, you can add a breakpoint (since breakpoints apply to all threads). The `eval` command can be used to evaluate expression in another thread. For instance, `eval 7 $i` will evaluate the `$i` in thread 7. The `ls` command can show lexical variables in another thread. Note that only variables in the innermost lexical scope will be shown. The `defer` command can be used to switch to another stopped thread. Here is an example: ### Switching between threads When multiple threads are stopped, `defer` will switch from one to another. For instance, the example below has `eg/defer.raku`, with threads 7-11 stopped, as well as thread 1. After looking at the stack in thread 1, typing `d 8` will change to thread 8, and subsequent commands will run in the context of that thread. (also note that if a lot of threads are stopped at the same line of code, they are shown in a footnote) dawa [1]> w --- current stack --- in block at eg/defer.raku line 16 -- current location -- 4 │ │ start { 5 │ │ my $x = 10; 6 │ │ loop { 7 │ │ stop; 8 │ (a) │ $x++; 9 │ │ } 10 │ │ } 11 │ │ } 12 │ │ 13 │ │ my $y = 100; 14 │ │ loop { 15 │ │ stop; 16 │ [1]▶ │ say "y is $y"; 17 │ │ $y += 111; 18 │ │ last if $y > 500; 19 │ │ } 20 │ │ ──────────────────────────────────────── (a) : [8][9][7][10][11] ──────────────────────────────────────── dawa [1]> d 8 8 ▶ $x++; dawa [8]> w --- current stack --- in block at eg/defer.raku line 8 -- current location -- 4 │ │ start { 5 │ │ my $x = 10; 6 │ │ loop { 7 │ │ stop; 8 │ (a)▶ │ $x++; 9 │ │ } 10 │ │ } 11 │ │ } 12 │ │ 13 │ │ my $y = 100; 14 │ │ loop { 15 │ │ stop; 16 │ [1] │ say "y is $y"; 17 │ │ $y += 111; 18 │ │ last if $y > 500; 19 │ │ } 20 │ │ ──────────────────────────────────────── (a) : [8][9][7][10][11] ──────────────────────────────────────── dawa [8]> $x 10 ## ABOUT THE NAME The word `dawa` can refer to either medicine or poison in Swahili. In the latter sense, it would be used to describe bug spray, i.e. a debugger -- but hopefully it'll also help be a cure for any ailments in your programs. ## SEE ALSO 1. There is a built-in `repl` statement, which will pause execution and start a repl loop at the current location. 2. [rakudo-debugger](https://github.com/jnthn/rakudo-debugger) provides a separate executable for debugging. Techniques there inspired this module. ## ENVIRONMENT The readline history is stored in `DAWA_HISTORY_FILE`, ~/.dawa-history by default. ## BUGS Since this relies on undocumented behavior, it could break at any time. Modifying the AST may also cause your program to behave in unexpected ways. It may be possible to improve this once the AST work in Raku is available. The `stop` routine won't work if it is the last statement in a file. There are probably other bugs -- let me know and send a patch! Also a mailing list is available to discuss features and send patches: https://lists.sr.ht/~bduggan/raku-dawa ## AUTHOR Brian Duggan (bduggan at matatu.org)