WIP/RFC: print stacktrace when eval fails

[mortenpi]

Currently, suppose I have a file like

function foo(x)
  error("error on line $(@__LINE__)")
end

#-

foo(42)

I get an error which does not actually show a stacktrace of where the exception was raised

julia> Literate.notebook("stacktrace.jl")
[ Info: generating notebook from `~/Julia/Literate/stacktrace.jl`
[ Info: executing notebook `stacktrace.ipynb`
┌ Error: error when executing notebook based on input file: `~/Julia/Literate/stacktrace.jl`
└ @ Literate ~/Julia/Literate/src/Literate.jl:667
ERROR: LoadError: error on line 2
in expression starting at string:1
when executing the following code block in file `~/Julia/Literate/stacktrace.jl`

```julia
foo(42)
```

Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] execute_block(sb::Module, block::String; inputfile::String)
   @ Literate ~/Julia/Literate/src/Literate.jl:785
 [3] execute_notebook(nb::Dict{Any, Any}; inputfile::String)
   @ Literate ~/Julia/Literate/src/Literate.jl:683
 [4] (::Literate.var"#36#38"{Dict{String, Any}})()
   @ Literate ~/Julia/Literate/src/Literate.jl:664
 [5] cd(f::Literate.var"#36#38"{Dict{String, Any}}, dir::String)
   @ Base.Filesystem ./file.jl:106
 [6] jupyter_notebook(chunks::Vector{Literate.Chunk}, config::Dict{String, Any})
   @ Literate ~/Julia/Literate/src/Literate.jl:663
 [7] notebook(inputfile::String, outputdir::String; config::Dict{Any, Any}, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Literate ~/Julia/Literate/src/Literate.jl:600
 [8] notebook (repeats 2 times)
   @ ~/Julia/Literate/src/Literate.jl:596 [inlined]
 [9] top-level scope
   @ REPL[6]:1

The stacktrace traces back to this error call:

Literate.jl/src/Literate.jl

Lines 785 to 792 in 42f3941

	error("""
	$(sprint(showerror, c.value))
	when executing the following code block in file `$(Base.contractuser(inputfile))`
	```julia
	$block
	```
	""")

With this patch you get something like the following (which could be much longer if the error thrown is deep in some dependency, which is the information you would want):

julia> Literate.notebook("stacktrace.jl")
[ Info: generating notebook from `~/Julia/Literate/stacktrace.jl`
[ Info: executing notebook `stacktrace.ipynb`
┌ Error: error when executing notebook based on input file: `~/Julia/Literate/stacktrace.jl`
└ @ Literate ~/Julia/Literate/src/Literate.jl:667
ERROR: Literate.EvalException: LoadError when executing code in: ~/Julia/Literate/stacktrace.jl
LoadError: error on line 2
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:33
  [2] foo(x::Int64)
    @ Main.##257 ./string:2
  [3] top-level scope
    @ string:1
  [4] eval
    @ ./boot.jl:360 [inlined]
  [5] include_string(mapexpr::typeof(identity), mod::Module, code::String, filename::String)
    @ Base ./loading.jl:1094
  [6] include_string (repeats 2 times)
    @ ./loading.jl:1104 [inlined]
  [7] #42
    @ ~/Julia/Literate/src/Literate.jl:781 [inlined]
  [8] (::IOCapture.var"#3#5"{Core.TypeofBottom, Literate.var"#42#43"{Module, String}, Task, Pipe, Base.TTY, Base.TTY})()
    @ IOCapture ~/.julia/packages/IOCapture/g8FZl/src/IOCapture.jl:105
  [9] with_logstate(f::Function, logstate::Any)
    @ Base.CoreLogging ./logging.jl:491
 [10] with_logger
    @ ./logging.jl:603 [inlined]
 [11] capture(f::Literate.var"#42#43"{Module, String}; rethrow::Type, color::Bool)
    @ IOCapture ~/.julia/packages/IOCapture/g8FZl/src/IOCapture.jl:103
 [12] execute_block(sb::Module, block::String; inputfile::String)
    @ Literate ~/Julia/Literate/src/Literate.jl:780
in expression starting at string:1

Stacktrace:
 [1] execute_block(sb::Module, block::String; inputfile::String)
   @ Literate ~/Julia/Literate/src/Literate.jl:801
 [2] execute_notebook(nb::Dict{Any, Any}; inputfile::String)
   @ Literate ~/Julia/Literate/src/Literate.jl:683
 [3] (::Literate.var"#36#38"{Dict{String, Any}})()
   @ Literate ~/Julia/Literate/src/Literate.jl:664
 [4] cd(f::Literate.var"#36#38"{Dict{String, Any}}, dir::String)
   @ Base.Filesystem ./file.jl:106
 [5] jupyter_notebook(chunks::Vector{Literate.Chunk}, config::Dict{String, Any})
   @ Literate ~/Julia/Literate/src/Literate.jl:663
 [6] notebook(inputfile::String, outputdir::String; config::Dict{Any, Any}, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ Literate ~/Julia/Literate/src/Literate.jl:600
 [7] notebook (repeats 2 times)
   @ ~/Julia/Literate/src/Literate.jl:596 [inlined]
 [8] top-level scope
   @ REPL[2]:1

Now, I don't yet quite have a good idea of how to do this well, so opening this for discussion. This was just a side tangent when working on a notebook. A few notes though:

• The simple option (least amount of code modified) would actually be to sprintf the backtrace into error. However, returning a custom exception object seems like the more correct thing to do.
• The @error from the parent function seems actually redundant.
• I noticed that by using include_string, any error I think gets wrapped in LoadError (but this does not seem to be documented). It also adds a bunch of things into the stacktrace which are hard to get rid of.
• It's sorta confusing that we print two stacktraces.. but it's sorta the right thing to do. Unless we can somehow do some rethrow-type magic..

[mortenpi]

This was a naive attempt the get rid of the stackframes introduced by include_string, but it didn't quite work right.