Overload print for Markdown Strings

[cortner]

@one-more-minute

It would be nice to create Base.Markdown.MD objects and then output them using print or println. They could then be used as messages, or even error messages.

See also discussion on julia-users.

[stevengj]

Why not just use display(x) on x::MD? Assuming there is a display backend that knows how to use writemime(io, "text/markdown", x) output (or MD defines a text/html output), then it will display correctly ... rich output is the whole point of the display stuff.

print(io, x) is essentially a synonym for writemime(io, "text/plain", x) so it doesn't seem appropriate here.

[MikeInnes]

When Markdown is displayed in a terminal (via display) it handles other bits of formatting like margins and trailing newlines. The idea would be for print(::MD) to have slightly different semantics to that so that you can intersperse printing plain and formatted text to the terminal.

[stevengj]

What would be the semantics of print(::MD)?

[cortner]

For most of my own uses, display would actually work ok - to be honest I just wasn't aware of it when I opened this issue. What about error messages though? I thought it would also be able to write

    error(md"Error with `code` or *other formatting*?")

but at the moment the output will be a discription of the MD object. Which command is used here?

[stevengj]

error throws an ErrorException, which is currently parameterized by a string. It doesn't actually display anything. To allow markdown in error messages, you would need:

• Modify the ErrorException type (and similar exceptions) to accept an arbitrary object (or some set of types) as an exception message.
• Modify the showerror (IIRC) function in the REPL to do richly formatted backtrace messages.
• IPython uses a special pyerr/error message type to display backtraces, and in IPython 3 I think this will accept richly formatted tracebacks; we just have to use it.

[cortner]

I can play around with this and report back, but is there any point in pursuing this, or would I be the only one interested in features like this?

[stevengj]

I suspect that it is probably premature to add markdown messages to exceptions; that would be a fairly intrusive change since exceptions are used everywhere.

[MikeInnes]

You don't have to change the ErrorException type – you could just overload error to respond to MD types by creating a different RichException or similar type. It would be a completely non-intrusive change.

[stevengj]

@one-more-minute, that's true; a much cleaner solution.

[stevengj]

Looks like the Markdown display should be updated. Currently, it overloads Base.display(d::Base.REPL.REPLDisplay, md::MD) to decide whether to print ANSI terminal color codes etcetera, when nowadays it should use IOContext for this in show.

Then you will be able to just call show with an appropriate IOContext if you want to print the same thing to any stream.