Julia throws additional LoadError during exception handling

General Usage
,
1

I’m not really sure if this is the right place for this question, so please let me know where I should be if this is not the right place.

I’m new to Julia, and I am writing some scripts that I’d like to use from the Linux command line. This works all fine, but when I included some exception handling into my script I found that that whenever I throw an exception Julia throws an additional LoadError So for example if I run a script containing only the following line:

throw(DivideError())

I get the both a DivideError (which I want) and a LoadError (Which I don’t want) when I run the script:

$ julia throwError.jl
ERROR: LoadError: DivideError: integer division error
Stacktrace:
 [1] top-level scope at /path/throwError.jl:1
 [2] include(::Function, ::Module, ::String) at ./Base.jl:380
 [3] include(::Module, ::String) at ./Base.jl:368
 [4] exec_options(::Base.JLOptions) at ./client.jl:296
 [5] _start() at ./client.jl:506
in expression starting at /path/throwError.jl:1

Can somebody explain what is happening here, and how I can prevent the LoadError?

2 Likes
2

Hi Jonas! You provided a concise example, which makes it so much easier to help.

Julia runs the code in order to load it. If you put the throw() inside a function, then Julia will run the function definition, but it won’t run the function that gets defined. You won’t see the DivideError until later, when you call the function. So:

function suspicious_fry(x)
    throw(DivideError())
    x
end
2 Likes
3

in short, this is happening because you’re throwing in global scope, which probably is not a realistic scenario.

4

Hi aldogert,

Thank you for your answer. It is not so much that I have a problem, it’s more that I try to understand how Julia works. For me throwing exceptions works fine. I was just wondering why the DivideError is wrapped by a LoadError.

Putting the throw() inside a function changes the stacktrace slightly but doesn’t change the error that is thrown. I asked this question as well on stackoverflow and somebody there pointed me to this pull request on github. What I understood from it is that Julia wraps exception throws in a LoadError as a way to show the stacktrace, but that they hope to deprecate this behavior at some point by improving backtracing in general.

But maybe somebody who has a better understanding of this, can phrase it more clearly than me.

5

Hi Jonas. Maybe another way to understand this is to think about the Julia code for include(). It would look like this:

function include(filename)
    open(filename) do ofile
        for line in ofile
            try
                run(line)
            catch y
                throw(LoadError(y))
            end
        end
    end
end

So run(line) creates a DivideError, and then it gets caught and passed to the user as the reason loading wasn’t possible.

1 Like
7

Hi Aldogert, Thanks for your code example. It made things more me clear for me.

However, I still think its peculiar that the catch y throw(LoadError(y)) is/was necessary to show the line numbers in the stack trace (at least that is what I understood from the PR).

1 Like
8

How about this, two files:

second.jl throws the exception only if the global run flag is set:

function panic_at_the_disco()
    throw(DivideError())
end

if run == true
    panic_at_the_disco()
end

The main file (first.jl):

run = false
include("second.jl")

try
    println("*******************************")
    println("Calling panic...")
    panic_at_the_disco()
catch ex
    println("Exception caught: $(typeof(ex))")
    println()
end

try
    println("*******************************")
    println("Running panic during include...")
    global run = true
    include("second.jl")
catch ex
    println("Exception caught: $(typeof(ex))")
    if ex isa LoadError
        println("Caused by: $(typeof(ex.error))")
    end
    println()
end

Output of running first.jl:

julia> include("first.jl")
*******************************
Calling panic...
Exception caught: DivideError

*******************************
Running panic during include...
Exception caught: LoadError
Caused by: DivideError

So include() wraps any exception thrown when loading/running the file in a LoadError but you can get the original error by accessing ex.error.

1 Like