Add Julia REPL

[adrhill]

This is a draft PR to add syntax highlighting to the Julia REPL mode julia-repl.

What is it?

The Julia programming language has an interactive read–eval–print loop (REPL) that requires custom syntax highlighting. Julia's documentation tools distinguish between julia-repl and julia code blocks for markdown syntax highlighting.

• Julia Code vs. Julia REPL. Note the green julia> prompt and output:
  

• Example of Julia REPL syntax highlighting in the Julia documentation:
  

Why is it needed?

Since the REPL is used to install and demonstrate packages, julia-repl code blocks are widely used in GitHub READMEs, as seen in the 560 results here. Outside of READMEs, julia-repl is used more than 10.000 times in code.
However, GitHub doesn't recognize julia-repl markdown code blocks, only julia.

• julia codeblock:

  n = 1
  foo(x) = 2*x
  foo(n)

• julia-repl codeblock:

  julia> n = 1
  1
  
  julia> foo(x) = 2*x
  foo (generic function with 1 method)
  
  julia> foo(n)
  2

Description

Interestingly, a Julia REPL grammar already exists in linguist as julia.console:

• linguist's grammars.yml
• julia-console.cson

I wasn't sure on how to add this to languages.yml, since the Julia REPL mode isn't a programming language by itself and therefore doesn't have a custom file type.

I would love to get some guidance on how to get julia-repl markdown blocks working on GitHub.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on github.com.
    • Search results for each extension:
      • julia-repl is used in 560 READMEs: https://github.com/search?type=code&q=path%3AREADME.md+%60%60%60julia-repl
      • julia-repl is used more than 10.000 times in code: https://github.com/search?type=code&q=%60%60%60julia-repl
  • I have included a real-world usage sample for all extensions added in this
    • julia-repl doesn't have a custom file type
  • I have included a syntax highlighting grammar: [URL to grammar repo]
    • A Julia REPL grammar already exists in linguist as julia.console
      • linguist's grammars.yml
      • julia-console.cson
  • I have added a color
    • Hex value: #a270ba
    • Rationale: This is the same color Julia uses
  • I have updated the heuristics to distinguish my language from others using the same extension.
    • The Julia REPL mode doesn't have an extension.

[lildude]

This approach is good and similar to what was done in #6302 for Elvish transcripts.

You're also going to need to update the grammars README.md (the test will fail if you don't).

[adrhill]

The test failure says:

1) Failure:
TestLanguage#test_all_languages_have_a_valid_ace_mode [test/test_language.rb:422]:
The following languages do not have an Ace mode listed in languages.yml. Please add an Ace mode for all new languages.
If no Ace mode exists for a language, mark the language with `ace_mode: text` in lib/linguist/languages.yml.
Julia REPL 

Should I add ace_mode: text or ace_mode: julia?

[lildude]

Should I add ace_mode: text or ace_mode: julia?

Top of that file has your answer 😉

linguist/lib/linguist/languages.yml

Lines 9 to 11 in e2012cd

	# ace_mode - A String name of the Ace Mode used for highlighting whenever
	# a file is edited. This must match one of the filenames in https://gh.io/acemodes.
	# Use "text" if a mode does not exist.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.

[adrhill]

Awesome! Thanks for the guidance. :)

[adrhill]

Now online #6870 (comment)! 🥳

n = 1
foo(x) = 2*x
foo(n)

julia> n = 1
1

julia> foo(x) = 2*x
foo (generic function with 1 method)

julia> foo(n)
2