What is used for value display in REPL?

[aplavin]

My original question is "which function is used by the REPL to display values", and I couldn't really find a definite answer.

Docs point to display(x)

julia/doc/src/manual/types.md

Lines 1483 to 1484 in 6f8ba49

	where the single-line `show(io, z)` form is still used for an array of `Polar` values. Technically,
	the REPL calls `display(z)` to display the result of executing a line, which defaults to `show(stdout, MIME("text/plain"), z)`,

and to show

julia/stdlib/REPL/docs/src/index.md

Line 60 in 6f8ba49

Objects are printed at the REPL using the [`show`](@ref) function with a specific [`IOContext`](@ref).

on different pages.

But this is definitely not the whole story, the output of display(x) and just x in REPL is different:

julia> using StructArrays, JLArrays  # or any actual GPU array instead of JLArray

julia> x = StructArray(a=JLArray(rand(3)))
3-element StructArray(::JLArray{Float64, 1}) with eltype @NamedTuple{a::Float64}:
 (a = 0.7284120253405527,)
 (a = 0.12606390581511362,)
 (a = 0.6120157378655939,)

julia> x |> display
3-element StructArray(::JLArray{Float64, 1}) with eltype @NamedTuple{a::Float64}:
ERROR: Scalar indexing is disallowed.
...

So, would be nice to document how it actually works... I personally don't understand it for now.

[jariji]

See also

https://discourse.julialang.org/t/improving-doc-for-display-print-show-repr/69124

[stevengj]

This is documented in Custom pretty-printing in the manual.

Technically, the REPL calls display(z) to display the result of executing a line, which defaults to show(stdout, MIME("text/plain"), z), which in turn defaults to show(stdout, z)

The REPL calls display, which defaults to the 3-argument show (with an appropriate IOContext — that line of the manual should be fixed to note that the REPL wraps stdout in an IOContext — fixed in #53926), which defaults to the 2-argument show. (print calls the 2-argument show.)

However, GPUArrays.jl does some task-dependent enabling of scalar indexing which makes show behave differently in the REPL display. This is a bit confusing — I wish they used show overloading or the IOContext instead. (But obviously packages can overload these functions in all sorts of ways, and we can't control how closely they follow the Base documentation.)

[stevengj]

@jariji, see also #53927.

[stevengj]

#53926 fixes the omission of display from the REPL docs, and should close this issue.