Redefining show for Type{Foo{T}} breaks display of any MethodError

[tbreloff]

I think the issue is on this line

as it doesn't actually show any useful information.

[ihnorton]

@tbreloff: please provide a demonstrative code sample.

[ihnorton]

(if you are hitting that line: Julia tried to print the backtrace, but failed... there is nothing else to be printed at that point)

[tbreloff]

So then the real problem is that printing the backtrace failed, in which case the bug is somewhere else?

Here's the REPL output when try to show the object (which, in this case, is probably failing somewhere within Gadfly's draw method). As you can see it never shows a backtrace, so I'm not sure exactly where the problem is. Do you have an idea of where the backtrace might fail? I can try to debug myself.

julia> p
Error showing value of type Plots.Plot{Plots.ImmersePackage}:
ERROR: MethodError: `convert` has no method matching convert(::Type{ColorTypes.RGBA{Float32}}, ::Array{ColorTypes.RGB{T<:Union{AbstractFloat,FixedPointNumbers.FixedPoint}},1})
This may have arisen from a call to the constructor ColorTypes.RGBA{Float32}(...),
since type constructors fall back to convert methods.SYSTEM: show(lasterr) caused an error

[ihnorton]

Do you have an idea of where the backtrace might fail?

From that error message, it is probably in a show method. You could try printing each of the fields of that type until you find which one can't be shown. Then look at the overloads using @which, etc.

[tbreloff]

I'm going to close this until I can pinpoint if the issue lies within base julia. Thanks.

[yuyichao]

It would be helpful if you could at least provide a complete procedure to reproduce the issue.

[tbreloff]

So I peppered the Gadfly codebase with println's and show's in order to pinpoint where the error gets thrown, and now the backtraces are printing (within one of Gadfly's render methods). I wonder if there's some weird thing with STDERR flushing, and that maybe there's multiple writers hitting a race condition, and writing output removes the race? I'm not really sure how to investigate further.

[timholy]

That SYSTEM: show(lasterr) caused an error is worrisome. I've seen that one too but never pinned down something reproducible, except to say that I only see it after using Gtk. (This could be a red herring; it could be the fault of some other package, I just remember that Gtk was loaded.)

@tbreloff, I'll echo the requests of others: a full reproducible test case might help find it, even if it involves lots of packages.

[tbreloff]

Yes it could be related to Gtk! I can't remember if I have only had the
problems after loading Immerse... It's possible the problem went away
because I restarted and switched to Gadfly only. I'll investigate when I
have time.

On Thursday, September 24, 2015, Tim Holy notifications@github.com wrote:

That SYSTEM: show(lasterr) caused an error is worrisome. I've seen that
one too but never pinned down something reproducible, except to say that I
only see it after using Gtk. (This could be a red herring; It could be
the fault of some other package, I just remember that Gtk was loaded.)

@tbreloff https://github.com/tbreloff, I'll echo the requests of
others: a full reproducible test case might help find it, even if it
involves lots of packages.

—
Reply to this email directly or view it on GitHub
#13306 (comment).

[tbreloff]

It seems like the backtraces are not shown when Immerse (and hence Gtk) are loaded. The first pass will import Gadfly, Compose, and the second pass will import Immerse, Gadfly, Compose, Gtk. If I can figure out more detailed info I'll post it.

_
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.4.0-rc1+78 (2015-09-17 01:30 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 1af871b* (7 days old release-0.4)
|__/                   |  x86_64-redhat-linux

julia> using Plots; gadfly!()
[Plots.jl] Default backend: immerse
[Plots.jl] Switched to backend: gadfly
Plots.GadflyPackage()

julia> p = plot(rand(10), c=Any[distinguishable_colors(10)])
[Plots.jl] Initializing backend: gadfly
INFO: Recompiling stale cache file /home/tom/.julia/lib/v0.4/Gadfly.ji for module Gadfly.
INFO: Recompiling stale cache file /home/tom/.julia/lib/v0.4/Compose.ji for module Compose.
WARNING: `require` is deprecated, use `using` or `import` instead
 in depwarn at deprecated.jl:73
 [inlined code] from deprecated.jl:693
 in require at no file:0
 in isinstalled at /home/tom/.julia/v0.4/Compose/src/Compose.jl:36
 in include at ./boot.jl:261
 in include_from_node1 at ./loading.jl:304
 [inlined code] from none:2
 in anonymous at no file:0
 in process_options at ./client.jl:284
 in _start at ./client.jl:411
while loading /home/tom/.julia/v0.4/Compose/src/Compose.jl, in expression starting on line 155
WARNING: `require` is deprecated, use `using` or `import` instead
 in depwarn at deprecated.jl:73
 [inlined code] from deprecated.jl:693
 in require at no file:0
 in isinstalled at /home/tom/.julia/v0.4/Compose/src/Compose.jl:36
 in include at ./boot.jl:261
 in include_from_node1 at ./loading.jl:304
 [inlined code] from none:2
 in anonymous at no file:0
 in process_options at ./client.jl:284
 in _start at ./client.jl:411
while loading /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl, in expression starting on line 961
WARNING: Base.None is deprecated, use Union{} instead.
in aesthetics_type at /home/tom/.julia/v0.4/Gadfly/src/coord.jl:112
WARNING: Base.FloatingPoint is deprecated, use AbstractFloat instead.
in concrete_minimum at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:50
WARNING: Base.FloatingPoint is deprecated, use AbstractFloat instead.
in concrete_minimum at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:50
WARNING: Base.FloatingPoint is deprecated, use AbstractFloat instead.
in concrete_maximum at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:72
WARNING: Base.FloatingPoint is deprecated, use AbstractFloat instead.
in concrete_maximum at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:72
WARNING: Base.String is deprecated, use AbstractString instead.
in showoff at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:112
WARNING: Base.String is deprecated, use AbstractString instead.
in showoff at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:112
WARNING: Base.String is deprecated, use AbstractString instead.
in showoff at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:112
WARNING: Base.String is deprecated, use AbstractString instead.
in showoff at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:280
WARNING: Base.String is deprecated, use AbstractString instead.
in showoff at /home/tom/.julia/v0.4/Showoff/src/Showoff.jl:36
here: text/html
about to try_display 1
WARNING: Base.None is deprecated, use Union{} instead.
in aesthetics_type at /home/tom/.julia/v0.4/Gadfly/src/coord.jl:112
CT = ColorTypes.RGBA{Float32}
Error showing value of type Plots.Plot{Plots.GadflyPackage}:
ERROR: MethodError: `append!` has no method matching append!(::Array{ColorTypes.RGBA{Float32},1}, ::ColorTypes.RGBA{Float32})
Closest candidates are:
  append!{T}(::Array{T,1}, ::AbstractArray{T,1})
 in render at /home/tom/.julia/v0.4/Gadfly/src/geom/line.jl:196
 in render at /home/tom/.julia/v0.4/Gadfly/src/geometry.jl:47
 in render_prepared at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:863
 in render at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:802
 in display at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:1061
 [inlined code] from multimedia.jl:151
 in display at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:1002
 in display at /home/tom/.julia/v0.4/Plots/src/Plots.jl:129
 in display at REPL.jl:118
 [inlined code] from multimedia.jl:151
 in display at multimedia.jl:162
 in print_response at REPL.jl:135
 in print_response at REPL.jl:122
 in anonymous at REPL.jl:627
 in run_interface at ./LineEdit.jl:1610
 in run_frontend at ./REPL.jl:866
 in run_repl at ./REPL.jl:170
 in _start at ./client.jl:453

julia> 
[tom@tomoffice ~]$ julia
               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.4.0-rc1+78 (2015-09-17 01:30 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 1af871b* (7 days old release-0.4)
|__/                   |  x86_64-redhat-linux

julia> using Plots; immerse!()
[Plots.jl] Default backend: immerse
[Plots.jl] Switched to backend: immerse
Plots.ImmersePackage()

julia> p = plot(rand(10), c=Any[distinguishable_colors(10)])
[Plots.jl] Initializing backend: immerse
INFO: Recompiling stale cache file /home/tom/.julia/lib/v0.4/Immerse.ji for module Immerse.
WARNING: Method definition draw(Compose.Backend, Compose.Context) in module Compose at /home/tom/.julia/v0.4/Compose/src/container.jl:404 overwritten in module Immerse at /home/tom/.julia/v0.4/Immerse/src/compose.jl:33.
WARNING: Method definition display(Base.REPL.REPLDisplay, Base.Multimedia.MIME{:text/html}, Gadfly.Plot) in module Gadfly at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:1057 overwritten in module Immerse at /home/tom/.julia/v0.4/Immerse/src/display_gadfly.jl:144.
WARNING: Method definition display(Base.REPL.REPLDisplay{R<:Base.REPL.AbstractREPL}, Base.Multimedia.MIME{:text/html}, Gadfly.Plot) in module Gadfly at /home/tom/.julia/v0.4/Gadfly/src/Gadfly.jl:1057 overwritten in module Immerse at /home/tom/.julia/v0.4/Immerse/src/display_gadfly.jl:144.
WARNING: Method definition draw(Compose.Backend, Compose.Context) in module Compose at /home/tom/.julia/v0.4/Compose/src/container.jl:404 overwritten in module Immerse at /home/tom/.julia/v0.4/Immerse/src/compose.jl:33.
about to display
here: text/html
about to try_display 1
here
about to try_display 2
WARNING: Base.None is deprecated, use Union{} instead.
in aesthetics_type at /home/tom/.julia/v0.4/Gadfly/src/coord.jl:112
CT = ColorTypes.RGBA{Float32}
Error showing value of type Plots.Plot{Plots.ImmersePackage}:
ERROR: MethodError: `append!` has no method matching append!(::Array{ColorTypes.RGBA{Float32},1}, ::ColorTypes.RGBA{Float32})
Closest candidates are:
  append!{T}(::Array{T,1}, ::AbstractArray{T,1})
  append!(::Gtk.GtkContainer, ::Any)

julia> 

[timholy]

I hit this myself, and for @tbreloff's benefit I'll explain my debugging procedure. A couple of minutes of playing around led me to realize that

julia> using Gtk

julia> 5+"hello"
ERROR: MethodError: `+` has no method matching +(::Int64, ::ASCIIString)SYSTEM: show(lasterr) caused an error

was a way to consistently reproduce the bug. There is nothing particular about that choice of call, you just have to trigger a MethodError.

From there, I started commenting out various include statements in the Gtk package. This quickly traced it down to src/GLib/glist.jl. Then a "binary search" (commenting out half of the file and testing, then subdividing the responsible half in half again, etc.) narrowed it down to a single line.

From there, I was able to come up with a standalone way to reproduce this. Here it is:

julia> type Foo{T}
           data::T
       end

julia> Base.show{T}(io::IO, ::Type{Foo{T}}) = print(io, "This is Foo{T}")
show (generic function with 98 methods)

julia> 5+"hello"
ERROR: MethodError: `+` has no method matching +(::Int64, ::ASCIIString)SYSTEM: show(lasterr) caused an error

While @JeffBezanson has in the past said "don't alter the printing of types", this is still a julia bug.