display "(empty range)" in case of empty ranges

[prashantrahul141]

As mentioned in #40331, empty ranges could be confusing. This pr adds a message "(empty range)" incase if the range is empty.
So for the example given in the issue

julia> a = 1:3
1:3

julia> b = 5:6
5:6

julia> intersect(a,b)  # old behaviour
5:4

julia> intersect(a,b)  # new behaviour
5:4 (empty range)

[Seelengrab]

This is the 2-argument show, which should be parseable Julia code. See https://docs.julialang.org/en/v1/base/io-network/#Base.show-Tuple{IO,%20Any} .

[prashantrahul141]

would it be better to use compact in IOContext to control this like @mikmoore suggested here?

basically this

    if !get(io, :compact, false) && isempty(r)
        print(io, " (empty range)")
    end

[KristofferC]

I think it should be in a three arg show method.

[Seelengrab]

Yes, that's what I was getting at. From the docstring I linked:

For a more verbose human-readable text output for objects of type T, define show(io::IO, ::MIME"text/plain", ::T) in addition.

:compact showing is unrelated to this.

[prashantrahul141]

I see now. So I assume it would be better to define show(io::IO, ::MIME"text/plain", ::T) for range types rather than change the existing ones?

[mikmoore]

Can we tweak this so that it doesn't print when IOContext is compact?

And also try to minimize its interference with the ability to copy-paste to reconstruct an object (which might be addressed by the compact)? For example,

julia> print([1:2, 3:2, 4:5])
UnitRange{Int64}[1:2, 3:2, 4:5]

I can use UnitRange{Int64}[1:2, 3:2, 4:5] to reconstruct this object but UnitRange{Int64}[1:2, 3:2 (empty range), 4:5] is a ParseError.

[jishnub]

If I understand correctly, the issue is mainly with an empty UnitRange being confused with a StepRange with a step of -1? Do we need this for every range type?

[prashantrahul141]

I assume a better approach would be to define a 3-argument show (show(io::IO, ::MIME"text/plain", ::T)) for range types. As @jishnub mentioned, should it be defined for all range types or just for UnitRange?

I think it should be defined for all range types since 3-argument show is meant to display additional details and to ensure consistency.

[jishnub]

To take the LinRange example, here are the displayed forms for the 3 and 2-argument show methods:

julia> show(stdout, MIME"text/plain"(), LinRange(2.0, 4.5, 5))
5-element LinRange{Float64, Int64}:
 2.0, 2.625, 3.25, 3.875, 4.5

julia> show(stdout, LinRange(2.0, 4.5, 5))
LinRange{Float64}(2.0, 4.5, 5)

In the first case, the length is explicitly mentioned in the summary. In the second case, the last parameter corresponds to the length, although this isn't explicitly pointed out (as this needs to be runnable julia code). Since we're discussing the 3-argument case here, the displayed form seems to indicate the length adequately?

I feel the issue is mainly with ranges where the 3-argument show uses the 2-argument method for the actual display. Or, in other words, any range that does not specialize the 3-argument show method. LinRange and LogRange are ones where the 3-argument form does display the summary, so these may be excluded.

[prashantrahul141]

So I am trying to define a 3 argument show in base/range.jl, something like:

function Base.show(io::IO, ::MIME"text/plain", r::AbstractUnitRange{T}) where T
   # impl
end

and I am getting this

error during bootstrap:
LoadError(at "Base_compiler.jl" line 3: LoadError(at "range.jl" line 1119: LoadError(at "range.jl" line 1119: UndefVarError(var=:var"@MIME_str", world=0x0000000000000e4e, scope=Base))))
ijl_undefined_var_error at /home/prashant/Projects/julia-fork/src/rtutils.c:154
jl_eval_global_var at /home/prashant/Projects/julia-fork/src/interpreter.c:169 [inlined]
eval_value at /home/prashant/Projects/julia-fork/src/interpreter.c:223
jl_interpret_toplevel_expr_in at /home/prashant/Projects/julia-fork/src/interpreter.c:919
No code info - unknown interpreter state!

I am assuming this is because maybe MIME is not available at this stage of bootstraping. How can I get around this?

[nsajko]

not available at this stage of bootstraping. How can I get around this?

MIME and friends are included into Base here:

julia/base/Base.jl

Lines 63 to 64 in 885b1cd

	include("multimedia.jl")
	using .Multimedia

... so try inserting your method somewhere after that point, maybe try one of these files:

julia/base/Base.jl

Lines 112 to 113 in 885b1cd

	include("show.jl")
	include("arrayshow.jl")