Julia: Confusing error message when returning an empty symbol

[KristofferC]

julia> eval(:($(Symbol(""))))
ERROR: syntax: all-underscore identifier used as rvalue around REPL[30]:2

There is no all-underscore identifier used here.

Came up in https://github.com/JuliaDebug/Debugger.jl/issues/20 when trying to update the local variables in the frame:

julia> bar(x; y=false) = nothing;

julia> ci = @code_lowered Core.kwfunc(bar)((y = 1.0,), bar, 3.0);

julia> ci.slotnames
6-element Array{Any,1}:
 Symbol("#unused#")
 Symbol("#temp#")
 Symbol("")
 :x
 :y
 Symbol("#temp#")

which contains the Symbol("").

[StefanKarpinski]

There is no all-underscore identifier used here.

Isn't there? Which letter in "" is not an underscore? :trollface:

[JeffBezanson]

This is what happens when a mathematician writes a parser :joy:

[KristofferC]

Can easily be worked around on the Debugger.jl side so feel free to close if this is cba-to-care territory.

[JeffBezanson]

So, I don't fully know what's going on in Debugger.jl here, but eval'ing symbols from slotnames seems like a problem since those names aren't unique; they're basically debug info. For example this function has two :xs in its slotnames:

function f(x)
    let x = x
        x
    end
end

[StefanKarpinski]

Would it make sense to just change what is considered an "all-underscore identifier" and require it to contain at least one underscore, even though technically "" is all underscores?

[JeffBezanson]

Yes, sure. I imagine that code was written assuming empty symbols would not occur.

[KristofferC]

but eval'ing symbols from slotnames seems like a problem since those names aren't unique; they're basically debug info.

Is it strange for a debugger to use the debug info? And whole they aren't unique, we know which one of them that was last assigned.

[JeffBezanson]

By "debug info" I meant information for display, that is not part of the program's meaning. So it depends on how the debugger uses it. Anyway, I just wanted to call attention to that in case it was surprising.

[timholy]

Not a problem because they get two different symbolsslots:

julia> @code_lowered f(1)
CodeInfo(
1 ─ %1 = x@_2
│        x@_3 = %1
└──      return x@_3
)

JuliaInterpreter keeps a Dict called last_reference that holds the slot number to which x was last assigned.

@JeffBezanson, can I ask you to look at this related question? https://github.com/JuliaDebug/JuliaInterpreter.jl/pull/54#discussion_r260738406 Nvm, I think your example above was enough for me to figure this out.

[KristofferC]

This is part of the functionality where you can evaluate code (and thereby update variables in a frame that is being interpreter). The code that gets evaluated is something like

let x = x0, y = y0, z = z0...
    ret = <input code to be evaluated>
    ret, (x, y, z)
end

<update the slots with the new x, y, z>

where x, y, z are local variables with the values x0, y0, z0. In this frame it just happened that there was a local variable with the name Symbol("") that made this expression invalid.