Prototype interactive debugger for Julia
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Updated Last
1 Year Ago
Started In
August 2012

Debug.jl v0.1

Prototype interactive debugger for the Julia language. Bug reports and feature suggestions are welcome at The package also supports evaluation of expressions in local scope.


In julia, install the Debug package:


Interactive Usage

First, import the Debug package:

using Debug

Use the @debug macro to mark code that you want to be able to step through. Use the @bp macro to set a breakpoint -- interactive debugging will commence at the first breakpoint encountered. There is also a conditional version, e.g. @bp x>0 will break only when x>0. @debug can only be used in global (i.e. module) scope, since it needs access to all scopes that surround a piece of code to be analyzed.

The following single-character commands have special meaning:
h: display help text
s: step into
n: step over any enclosed scope
o: step out from the current scope
c: continue to next breakpoint
l [n]: list n source lines above and below current line, if source file information is available at this point (default n = 3)
p cmd: print cmd evaluated in current scope
q: quit debug session (calls error("interrupted"))
Anything else is parsed and evaluated in the current scope. To e.g. evaluate a variable named n, it can be entered with a space prepended.


Put the following in a file called example.jl:

using Debug
@debug function test()
    parts = {}
    for j=1:3
        for i=1:3
            push!(parts,"($i,$j) ")


Then, in the Julia terminal:

julia> include("example.jl")

at /home/toivo/.julia/Debug/test/example.jl:4

      3        parts = {}
 -->  4        @bp
      5        for j=1:3

debug:4> j
j not defined

debug:4> parts

debug:4> s

at /home/toivo/.julia/Debug/test/example.jl:5

      4        @bp
 -->  5        for j=1:3
      6            for i=1:3

debug:5> s

at /home/toivo/.julia/Debug/test/example.jl:6

      5        for j=1:3
 -->  6            for i=1:3
      7                push!(parts,"($i,$j) ")

debug:6> s

at /home/toivo/.julia/Debug/test/example.jl:7

      6            for i=1:3
 -->  7                push!(parts,"($i,$j) ")
      8            end

debug:7> i

debug:7> s

at /home/toivo/.julia/Debug/test/example.jl:7

      6            for i=1:3
 -->  7                push!(parts,"($i,$j) ")
      8            end

debug:7> i

debug:7> parts
{"(1,1) "}

debug:7> parts = {}

debug:7> o

at /home/toivo/.julia/Debug/test/example.jl:6

      5        for j=1:3
 -->  6            for i=1:3
      7                push!(parts,"($i,$j) ")

debug:6> j

debug:6> n

at /home/toivo/.julia/Debug/test/example.jl:6

      5        for j=1:3
 -->  6            for i=1:3
      7                push!(parts,"($i,$j) ")

debug:6> j

debug:6> push!(parts, "foo ")
{"(2,1) ","(3,1) ","(1,2) ","(2,2) ","(3,2) ","foo "}

debug:6> c

at /home/toivo/.julia/Debug/test/example.jl:10

      9        end
 -->  10       @bp
      11       println(parts...)

debug:10> parts 
{"(2,1) ","(3,1) ","(1,2) ","(2,2) ","(3,2) ","foo ","(1,3) ","(2,3) ","(3,3) "}

debug:10> c
(2,1) (3,1) (1,2) (2,2) (3,2) foo (1,3) (2,3) (3,3) 


Considerations for parallel code

The Debug package has not been written with support for parallel execution in mind. The instrumentation code that is inserted by the @debug macro will likely not work as intended for code that is sent to another process, or run in another thread. It might also cause trouble for Julia's serialization code, since the instrumentation code contains references to data structures with cycles.

For these reasons, it is recommended that if there is code inside of a @debug macro invocation that might be run in another thread or process to additionally wrap that code with the @notrap macro, and not use Debug features such as @localscope inside of it. This will ensure that no instrumentation is generated for the code in question and it will be passed through as is. Consequently, stepping through code wrapped in @notrap is not possible.

Experimental Features

Interpolations in entered code will currently be evaluated in the context of the Debug.Session module, before the expression itself is evaluated in the context of the current scope. Some of the debugger's internal state has been made available through this mechanism, and can be manipulated to influence debugging:
$n: The current node
$s: The current scope
$bp: Set{Node} of enabled breakpoints
$nobp: Set{Node} of disabled @bp breakpoints
$pre: Dict{Node} of grafts
Nodes refer to positions in the instrumented code, represented by nodes in the decorated AST produced from the original code.

Breakpoints can be manipulated using e.g.

$(push!(bp, n))    # set breakpoint at the current node
$(delete!(bp, n))  # unset breakpoint at the current node
$(push!(nobp, n))  # ignore @bp breakpoint at the current node

The above examples can also be written as e.g. $push!($bp, $n).
Code snippets can also be grafted into instrumented code. E.g.

$(pre[n] = :(x = 0))

will make the code x = 0 execute right before each execution of the current node.

Other nodes than the current node $n could be used in the examples above. Such nodes can be found by navigating from the current node, but the there is not much support for this yet.

Evaluation of code in local scope

By design, the Julia eval function only allows to evaluate code in global (i.e. module) scope. This allows for all kinds of optimizations, e.g. since the compiler can see all uses of all local variables.

The Debug package needs to be able to evaluate code in local scopes when it is entered at the debug prompt however; in fact this is the main functionality that the package provides, and can also be used as a standalone feature. To allow evaluation in a local scope, code is instrumented to create Scope objects, which contain getter and setter functions for each local variable accesible in a given scope.

Code wrapped inside the @debug macro can retrieve the current scope object using the @localscope macro. If minimal code instrumentation is desired, parts or all of the code wrapped in the @debug macro can be wrapped in the @notrap macro. The @notrap macro will disable stepping through the wrapped code, but will still allow the @localscope macro to be used. Scope objects will then only be created when entering scopes where they will be needed by some nested @localscope invocation.

Once a Scope object is available, local variables can be read and assigned in it by indexing with the corresponding symbols, and listed using keys(scope). Expressions can also be evaluated in a scope using the debug_eval function:

using Debug
@debug @notrap function f(x)
    outer = @localscope
    local inner
    pos = "outer"
    let pos = "inner"
        y = x
        inner = @localscope
    (outer, inner)

outer, inner = f(5)

@show keys(outer) keys(inner) # y is only present in inner

@show outer[:x] inner[:x]     # x is the same variable in both scopes
@show outer[:pos] inner[:pos] # pos refers to different variables in inner and outer

println("\nSetting `inner[:x] = 3`:")
inner[:x] = 3             # assigns to the single variable x
@show outer[:x] inner[:x] # both values have been updated

@show debug_eval(inner, :(x*x)) # evaluate an expression in the inner scope

which produces the output

keys(outer) => Set{Symbol}({:outer,:pos,:inner,:x})
keys(inner) => Set{Symbol}({:outer,:pos,:inner,:x,:y})

outer[:x] => 5
inner[:x] => 5
outer[:pos] => "outer"
inner[:pos] => "inner"

Setting `inner[:x] = 3`:
outer[:x] => 3
inner[:x] => 3

debug_eval(inner,:(x * x)) => 9

As seen above, it is possible to evaluate code in a local scope and to change the values of variables in it. It is however not possible to define new variables in a local scope, so the expression passed to debug_eval will be evaluated as if it were inside a let block in the given scope.

Custom Traps

There is an @instrument macro that works similarly to the @debug macro, but takes as first argument a trap function to be called at each expression that lies directly in a block. The example

using Base, Debug

firstline = -1
function trap(node::Node, scope::Scope) 
    global firstline = (firstline == -1) ? node.loc.line : firstline
    line = node.loc.line - firstline + 1
    print(line, ":")

    if (line == 2); debug_eval(scope, :(x += 1)) end

    if (line >  1); print("\tx = ", debug_eval(scope, :x)) end
    if (line == 3); print("\tk = ", debug_eval(scope, :k)) end

@instrument trap function f(n)
    x = 0       # line 1
    for k=1:n   # line 2
        x += k  # line 3
    end         # line 4
    x = x*x     # line 5
    x           # line 6


produces the output

2:	x = 1
3:	x = 1	k = 1
3:	x = 2	k = 2
3:	x = 4	k = 3
5:	x = 7
7:	x = 49

The scope argument passed to the trap function can be used with debug_eval(scope, ex) to evaluate an expression ex as if it were in that scope. @instrument in turn relies on the function Debug.Graft.instrument, which also allows to to specify at which nodes to add traps.

How it Works

The foundations of the Debug package is code for analyzing the scoping of symbols in a piece of code, and to modify code to allow one piece of code to be evaluated as if it were at some particular point in another piece of code. The interactive debug facility is built on top of this. The @debug macro triggers a number of steps:

  • The code passed to @debug is analyzed, and turned into a decorated AST built from nodes of type Debug.AST.Node. The format is almost identical to Julia's native AST format, but nodes also keep track of parent, static scope, and location in the source code.
  • The code is then instrumented to insert trap calls at each stepping point, entry/exit to scope blocks, etc. A Scope object that contains getter and setter functions for each visible local symbol is also created upon entry to each block that lies within a new environment.
  • The code passed to debug_eval is analyzed in the same way as to @debug. The code is then grafted into the supplied scope by replacing each read/write to a variable with a call to the corresponding getter/setter function, if it is visible at that point in the grafted code.

Known Issues

I have tried to encode the scoping rules of Julia as accurately as possible, but I'm bound to have missed something. Also,

  • The scoping rules for for blocks etc. in global scope are not quite accurate.
  • Code within macro expansions may become tagged with the wrong source file.

Known issues can also be found at the issues page. Bug reports and feature requests are welcome.