Strange display() error with Gadfly

[tomyun]

Normal behavior before using Gadfly:

julia> collect(())
0-element Array{Union{},1}

After importing Gadfly:

julia> using Gadfly

julia> collect(())
Error showing value of type Array{Union{},1}:
ERROR: MethodError: no method matching display(::Array{Union{},1})
Closest candidates are:
  display(::Any) at multimedia.jl:320
  display(::TextDisplay, ::MIME{Symbol("text/plain")}, ::Any) at multimedia.jl:240
  display(::TextDisplay, ::MIME, ::Any) at multimedia.jl:246
  ...
Stacktrace:
 [1] display(::Any) at ./multimedia.jl:330
 [2] #invokelatest#1 at ./essentials.jl:712 [inlined]
 [3] invokelatest at ./essentials.jl:711 [inlined]
 [4] print_response(::IO, ::Any, ::Bool, ::Bool, ::Any) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:161
 [5] print_response(::REPL.AbstractREPL, ::Any, ::Bool, ::Bool) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:146
 [6] (::REPL.var"#do_respond#38"{Bool,REPL.var"#48#57"{REPL.LineEditREPL,REPL.REPLHistoryProvider},REPL.LineEditREPL,REPL.LineEdit.Prompt})(::Any, ::Any, ::Any) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:729
 [7] (::REPL.var"#53#62")(::Any, ::Any, ::Vararg{Any,N} where N) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:997
 [8] #invokelatest#1 at ./essentials.jl:712 [inlined]
 [9] invokelatest at ./essentials.jl:711 [inlined]
 [10] (::REPL.LineEdit.var"#22#23"{REPL.var"#53#62",String})(::Any, ::Any) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/LineEdit.jl:1364
 [11] prompt!(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/LineEdit.jl:2446
 [12] run_interface(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/LineEdit.jl:2349
 [13] run_frontend(::REPL.LineEditREPL, ::REPL.REPLBackendRef) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:1055
 [14] run_repl(::REPL.AbstractREPL, ::Any) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:206
 [15] (::Base.var"#764#766"{Bool,Bool,Bool,Bool})(::Module) at ./client.jl:383
 [16] #invokelatest#1 at ./essentials.jl:712 [inlined]
 [17] invokelatest at ./essentials.jl:711 [inlined]
 [18] run_main_repl(::Bool, ::Bool, ::Bool, ::Bool, ::Bool) at ./client.jl:367
 [19] exec_options(::Base.JLOptions) at ./client.jl:305
 [20] _start() at ./client.jl:484

Seems like it has something do with GadflyDisplay pushed in Base.Multimedia.displays, but I'm not really familiar with how the display stack work. It's from Gadfly 1.3.0 on Julia 1.4.2.

[Mattriks]

The error is actually caused by Contour.jl. Contour.jl extends Base.show with 2 new methods for Arrays, which confuses what to do with collect(()). The following example illustrates this:

using Gadfly.Stat.Contour
x = collect(())
dsps = Base.Multimedia.displays
# Note typeof(dsps[1]) == Base.TextDisplay
display(dsps[1], x) 

MethodError: show(::Base.TTY, ::MIME{Symbol("text/plain")}, ::Array{Union{},1}) is ambiguous. Candidates:
  show(io::IO, ::MIME{Symbol("text/plain")}, c2s::Array{TC,1}) where TC<:Curve2 in Contour at C:\Users\mjf\.julia\packages\Contour\zVMve\src\Contour.jl:24
  show(io::IO, ::MIME{Symbol("text/plain")}, cls::Array{CL,1}) where CL<:ContourLevel in Contour at C:\Users\mjf\.julia\packages\Contour\zVMve\src\Contour.jl:33
Possible fix, define
  show(::IO, ::MIME{Symbol("text/plain")}, ::Array{Union{},1})

I suggest opening an issue at Contour.jl.

[Mattriks]

Moved to JuliaGeometry/Contour.jl#59