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How to use a custom theme with highlight (and reprex)

reprex::reprex_rtf() and reprex:::prex_rtf() shell out to highlight to produce rendered, syntax-highlighted R snippets as RTF. Normally I just choose one of the themes that ship with highlight, such as Darkbone or Solarized Light, for dark and light slides, respectively. But for this talk I was curious to see if I could create a custom theme that is more consistent with the Keynote theme.

  1. Find your existing themes. On my current system they are at /usr/local/Cellar/highlight/3.47/share/highlight/themes.

  2. Copy one of these files, such as darkbone.theme, to kuper.theme, where "kuper" is whatever you plan to call this custom theme. Modify (or not), in order to conduct a pilot study. For example, you might want to make at least one color obvious and garish, to easily confirm things are working.

  3. Specify the custom theme file via --config-file with a test .R file (such as a commented R file obtained via reprex_r(input = "abc.R", outfile = NA)):

     highlight abc_reprex_rendered.R --out-format rtf --no-trailing-nl \
   --encoding=UTF-8 --config-file kuper.theme > \
   abc_reprex_rendered.rtf

  4. Method 1 for making the custom theme available through --style on the command line: move the theme to ~/.highlight/themes/kuper.theme.

  5. Method 2: store theme in the relevant project, but symlink it to ~/.highlight/themes/kuper.theme.

  6. Method 3: symlink kuper.theme into the main themes directory.

  7. Regardless of method, you should now be able to specify the custom theme:

     highlight abc_reprex_rendered.R --out-format rtf --no-trailing-nl \
   --encoding=UTF-8 --style kuper > abc_reprex_rendered.rtf

  8. To use the custom theme with reprex, specify it the usual way via the reprex.highlight.hl_style option:

     options(reprex.highlight.hl_style = "kuper")

Details of a highlight theme

A theme file looks like this

Description = "custom theme for blah blah"

Default        = { Colour="#000000" }
Canvas         = { Colour="#ffffff" }
Number         = { Colour="#000000" }
Escape         = { Colour="#bd8d8b" }
String         = { Colour="#bd8d8b" }
StringPreProc  = { Colour="#bd8d8b" }
BlockComment   = { Colour="#ac2020", Italic=true }
PreProcessor   = { Colour="#000000" }
LineNum        = { Colour="#555555" }
Operator       = { Colour="#000000" }
LineComment = BlockComment

Keywords = {
 { Colour= "#9c20ee", Bold=true },
 { Colour= "#208920" },
 { Colour= "#0000ff" },
 { Colour= "#000000" },
}

Here's a visual look at Darkbone:

And same for "kuper", the custom theme for this talk:

Explanation:

  • Default is used for variable names, package names, and function arguments.
  • Canvas doesn't matter for reprex usage but indicates whether theme is designed for a dark or light background. It controls the background of a resulting .rtf file.
  • Number and String are for numbers and strings and are often the same.
  • Escape is for escaped characters, like the \n in a string.
  • LineComment is for comments and, in the case of a rendered reprex, R output.
  • Operator is for (, the assignment operator, ::, etc.
  • I haven't fiddled with BlockComment, StringPreProc, PreProcessor, LineNum.
  • You can set one color to another, previous one, by name. Example: LineNum = BlockComment.
  • Keyword group 1: Things like function(), if(), and for().
  • Keyword group 2: Special constants like NULL, NA, and TRUE.
  • Keyword group 3: isn't actually defined in r.lang, so I don't know
  • Keyword group 4: Function names.

r.lang

Found at /usr/local/Cellar/highlight/3.47/share/highlight/langDefs/r.lang.

Description="R"

Identifiers=[[ [a-zA-Z_\.][\w]* ]]

Keywords={
  { Id=1,
    List={"if", "else", "repeat", "while", "function", "for", "in", "next", "break", "ifelse", "switch"},
  },
  { Id=2,
    List={"NULL", "NA", "Inf", "NaN", "TRUE", "T", "FALSE", "F"},
  },
  { Id=4,
    Regex=[[([\w+_\.]+)\s*\(]],
  }  
}

Strings={
  Delimiter=[["|']],
}

Comments={
  { Block=false,
    Delimiter= { [[#]] },
  },
}

Operators=[[\(|\)|\[|\]|\{|\}|\,|\;|\:|\&|<|>|\!|\=|\/|\*|\%|\+|\-|\~|\|]]

EnableIndentation=true

Official docs

http://www.andre-simon.de/doku/highlight/en/highlight.php#ch3_4