[Github-comments] [geany/geany] Add Prolog filetype support (PR #3171)

[Robert Di Pardo]

> It's not something we need to implement - it's already implemented by the lexer.

To be exact, underscores denote an *unused* variable (called a "singleton"). The lexer [maps them][0] to `SCE_VISUALPROLOG_ANONYMOUS`:

~~~cpp
  } else if (sc.Match('_')) {
      sc.SetState(SCE_VISUALPROLOG_ANONYMOUS);
~~~

Only words in proper title-case [are styled][1] as `SCE_VISUALPROLOG_VARIABLE`:

~~~cpp
  } else if (isUpperLetter(sc.ch)) {
      sc.SetState(SCE_VISUALPROLOG_VARIABLE);
~~~

![vpl-singleton](https://user-images.githubusercontent.com/59004801/165650011-e7d48c7f-b1b7-4c5b-888d-746c49d6d18b.png)


[0]: https://github.com/ScintillaOrg/lexilla/blob/b7b401a2b169aa58dffa96888aca96b38c292a3c/lexers/LexVisualProlog.cxx#L431-L432
[1]: https://github.com/ScintillaOrg/lexilla/blob/b7b401a2b169aa58dffa96888aca96b38c292a3c/lexers/LexVisualProlog.cxx#L429-L430


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