Stream: helpdesk (published)

Topic: Confusion with MIME types as function arguments

mbaz (Mar 30 2021 at 17:56):

I've always been confused by this...
In https://docs.julialang.org/en/v1.6/manual/types/#man-custom-pretty-printing, this function to pretty-print polar numbers is defined:

Base.show(io::IO, ::MIME"text/html", z::Polar{T}) where {T} =
           println(io, "<code>Polar{$T}</code> complex number: ",
                   z.r, " <i>e</i><sup>", z.Θ, " <i>i</i></sup>")

and then it is called as

show(stdout, "text/html", Polar(3.0,4.0))

Why does "text/html", which is a String, match the signature ::MIME"text/html"?

If I try to do the same in my own function, it doesn't work:

julia> ttt(::MIME"text/html") = println("yes")
julia> ttt("text/html")
ERROR: MethodError: no method matching ttt(::String)
Closest candidates are:
  ttt(::MIME{Symbol("text/html")}) at REPL[14]:1
Stacktrace:
 [1] top-level scope
   @ REPL[16]:1

mbaz (Mar 30 2021 at 18:18):

Nevermind, I found it in multimedia.jl:
show(io::IO, m::AbstractString, x) = show(io, MIME(m), x)

mbaz (Mar 30 2021 at 18:19):

That part of the manual sure is confusing.

Mason Protter (Mar 31 2021 at 00:47):

Yeah. IMO the way we handle show has a lot of problems. It's really useful because it gives tonnes of different ways to customize show, but it sucks because it's really confusing and filled with edge cases.

mbaz (Mar 31 2021 at 01:17):

Yes; things like the two-argument and three-argument show variants, the reliance on MIME, plus display, plus all the multimedia I/O stuff are confusing. I spent hours getting Gaton's show to work properly in Pluto and IJulia, and I'm still not sure my implementation is correct.

Kirill Simonov (Mar 31 2021 at 20:43):

I submitted an issue regarding show() for containers, got no response so far... https://github.com/JuliaLang/julia/issues/40030

Last updated: Oct 02 2023 at 04:34 UTC