# JuliaInterpreter

This package implements an interpreter for Julia code. Normally, Julia compiles your code when you first execute it; using JuliaInterpreter you can avoid compilation and execute the expressions that define your code directly. Interpreters have a number of applications, including support for stepping debuggers.

## Use as an interpreter

Using this package as an interpreter is straightforward:

julia> using JuliaInterpreter

julia> list = [1, 2, 5]
3-element Vector{Int64}:
1
2
5

julia> sum(list)
8

julia> @interpret sum(list)
8

## Breakpoints

You can interrupt execution by setting breakpoints. You can set breakpoints via packages that explicitly target debugging, like Juno, Debugger, and Rebugger. But all of these just leverage the core functionality defined in JuliaInterpreter, so here we'll illustrate it without using any of these other packages.

Let's set a conditional breakpoint, to be triggered any time one of the elements in the argument to sum is bigger than 4:

julia> bp = @breakpoint sum([1, 2]) any(x->x>4, a);

Note that in writing the condition, we used a, the name of the argument to the relevant method of sum. Conditionals should be written using a combination of argument and parameter names of the method into which you're inserting a breakpoint; you can also use any globally-available name (as used here with the any function).

Now let's see what happens:

julia> @interpret sum([1,2,3])  # no element bigger than 4, breakpoint should not trigger
6

julia> frame, bpref = @interpret sum([1,2,5])  # should trigger breakpoint
(Frame for sum(a::AbstractArray; dims, kw...) in Base at reducedim.jl:873
c 1* 873  1 ─      nothing
2  873  │   %2 = ($(QuoteNode(NamedTuple)))() 3 873 │ %3 = Base.pairs(%2) ⋮ a = [1, 2, 5], breakpoint(sum(a::AbstractArray; dims, kw...) in Base at reducedim.jl:873, line 873)) frame is described in more detail on the next page; for now, suffice it to say that the c in the leftmost column indicates the presence of a conditional breakpoint upon entry to sum. bpref is a reference to the breakpoint of type BreakpointRef. The breakpoint bp we created can be manipulated at the command line julia> disable(bp) julia> @interpret sum([1,2,5]) 8 julia> enable(bp) julia> @interpret sum([1,2,5]) (Frame for sum(a::AbstractArray; dims, kw...) in Base at reducedim.jl:873 c 1* 873 1 ─ nothing 2 873 │ %2 = ($(QuoteNode(NamedTuple)))()
3  873  │   %3 = Base.pairs(%2)
⋮
a = [1, 2, 5], breakpoint(sum(a::AbstractArray; dims, kw...) in Base at reducedim.jl:873, line 873))

disable and enable allow you to turn breakpoints off and on without losing any conditional statements you may have provided; remove allows a permanent removal of the breakpoint. You can use remove() to remove all breakpoints in all methods.

@breakpoint allows you to optionally specify a line number at which the breakpoint is to be set. You can also use a functional form, breakpoint, to specify file/line combinations or that you want to break on entry to any method of a particular function. At present, note that some of this functionality requires that you be running Revise.jl.

It is, in addition, possible to halt execution when otherwise an error would be thrown. This functionality is enabled using break_on and disabled with break_off:

julia> function f_outer()
println("before error")
f_inner()
println("after error")
end;

julia> f_inner() = error("inner error");

julia> break_on(:error)

julia> fr, pc = @interpret f_outer()
before error
(Frame for f_outer() in Main at none:1
1  2  1 ─      Base.println("before error")
2* 3  │        f_inner()
3  4  │   %3 = Base.println("after error")
4  4  └──      return %3
callee: f_inner() in Main at none:1, breakpoint(error(s::AbstractString) in Base at error.jl:35, line 35, ErrorException("inner error")))

julia> leaf(fr)
Frame for error(s::AbstractString) in Base at error.jl:35
1  35  1 ─ %1 = (\$(QuoteNode(ErrorException)))(s)
2* 35  │   %2 = Core.throw(%1)
3  35  └──      return %2
s = "inner error"
caller: f_inner() in Main at none:1

julia> typeof(pc)
BreakpointRef

julia> pc.err
ErrorException("inner error")

julia> break_off(:error)

julia> @interpret f_outer()
before error
ERROR: inner error
Stacktrace:
[...]

Finally, you can set breakpoints using @bp:

julia> function myfunction(x, y)
a = 1
b = 2
x > 3 && @bp
return a + b + x + y
end
myfunction (generic function with 1 method)

julia> @interpret myfunction(1, 2)
6

julia> @interpret myfunction(5, 6)
(Frame for myfunction(x, y) in Main at none:1
⋮
3  4  │   %3 = x > 3
4  4  └──      goto #3 if not %3
b 5* 4  2 ─      nothing
6  4  └──      goto #3
7  5  3 ┄ %7 = a + b + x + y
⋮
x = 5
y = 6
b = 2
a = 1, breakpoint(myfunction(x, y) in Main at none:1, line 4))

Here the breakpoint is marked with a b indicating that it is an unconditional breakpoint. Because we placed it inside the condition x > 3, we've achieved a conditional outcome.

When using @bp in source-code files, the use of Revise is recommended, since it allows you to add breakpoints, test code, and then remove the breakpoints from the code without restarting Julia.

## debug_command

You can control execution of frames via debug_command. Authors of debugging applications should target debug_command for their interaction with JuliaInterpreter.

## Hooks

Consider if you were building a debugging application with a GUI component which displays a dot in the text editor margin where a breakpoint is. If a user creates a breakpoint not via your GUI, but rather via a command in the REPL etc. then you still wish to keep your GUI up to date. How to do this? The answer is hooks.

JuliaInterpreter has experimental support for having a hook, or callback function invoked whenever the set of all breakpoints is changed. Hook functions are setup by invoking the JuliaInterpreter.on_breakpoints_updated function.

To return to our example of keeping GUI up to date, the hooks would look something like this:

using JuliaInterpreter
using JuliaInterpreter: AbstractBreakpoint, update_states!, on_breakpoints_updated

breakpoint_gui_elements = Dict{AbstractBreakpoint, MarginDot}()
# ...
function breakpoint_gui_hook(::typeof(breakpoint), bp::AbstractBreakpoint)
bp_dot = MarginDot(bp)
draw(bp_dot)
breakpoint_gui_elements[bp] = bp_dot
end

function breakpoint_gui_hook(::typeof(remove), bp::AbstractBreakpoint)
bp_dot = pop!(breakpoint_gui_elements, bp)
undraw(bp_dot)
end

function breakpoint_gui_hook(::typeof(update_states!), bp::AbstractBreakpoint)
is_enabled = bp.enabled[]
bp_dot = breakpoint_gui_elements[bp]
set_fill!(bp_dot, is_enabled ? :blue : :grey)
end

on_breakpoints_updated(breakpoint_gui_hook)