Using Go templates to customize the ruler

[joelim-work]

From discussion in #2057

Currently the ruler at the bottom can be customized using the statfmt and rulerfmt options. While these options contain some level of flexibility, there are still some limitations when it comes to customization:

• Placeholders were substituted as-is, and there was no way to manipulate the result of the substitution. For example there was no way to display an alternate text if a placeholder expanded to an empty string.
• Colors/styles could only be applied on the result of an entire expansion, and it wasn't possible to color/style part of an expansion.
• There was no support for specifying field widths, (e.g. pad with spaces).
• The statfmt could not be drawn on the right side of the screen, and similarly rulerfmt components could not be drawn on the left side of the screen.

The PR #2083 changes this to use Go templates instead to provide greater flexibility for customization, with the syntax being closer to a programming language instead of a format string. The tradeoff for the extra customizability is that the configuration is now more complex, which increases the barrier of entry for users.

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To illustrate the history of the ruler configation:

1. Originally the ruler was fixed and there was no opportunity for users to customize its appearance
2. Then the ruler option was added in Customizable statusline, df #1168, which allowed users to choose which fields to display and in which order:

    set ruler 'acc:progress:selection:ind'

3. Then it was replaced with rulerfmt in Add rulerfmt option for improved ruler customization #1386 to allow for custom formatting:

   set rulerfmt "%a|  %p|  \033[7;31m %m \033[0m|  \033[7;33m %c \033[0m|  \033[7;35m %s \033[0m|  \033[7;34m %f \033[0m|  %i/%t"

4. And now the ruler file looks like this:

   {{if .Stat}}
   {{.ESC}}[36m{{.Stat.Permissions}}{{.ESC}}[0m
   {{if .Stat.LinkCount}} {{.Stat.LinkCount}}{{end}}
   {{if .Stat.User}} {{.Stat.User}}{{end}}
   {{if .Stat.Group}} {{.Stat.Group}}{{end}}
    {{humanize .Stat.Size | printf "%5s"}}
    {{.Stat.ModTime.Format .Options.timefmt}}
   {{if .Stat.Target}} -> {{.Stat.Target}}{{end}}
   {{end}}
   {{.SPACER}}
     {{.Acc}}
   {{if .Progress}}  {{join .Progress " "}}{{end}}
   {{if .Copy}}  {{.ESC}}[7;33m {{len .Copy}} {{.ESC}}[0m{{end}}
   {{if .Cut}}  {{.ESC}}[7;31m {{len .Cut}} {{.ESC}}[0m{{end}}
   {{if .Select}}  {{.ESC}}[7;35m {{len .Select}} {{.ESC}}[0m{{end}}
   {{if .Visual}}  {{.ESC}}[7;36m {{len .Visual}} {{.ESC}}[0m{{end}}
   {{if .Filter}}  {{.ESC}}[7;34m {{join .Filter " "}} {{.ESC}}[0m{{end}}
     {{.Index}}/{{.Total}}

Each step allows for greater flexibility in customization, but at the same time results in more complex syntax.

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Another suggestion would be to use a scripting language like Lua for handling the configuration, but then that requires using some 3rd party library (as opposed to the standard library), and the implementation would be even more complex as the API has to bridge between Lua and Go. I am probably not interested in taking things that far.

Anyway this is an idea that I want to suggest, hopefully it is not too confusing for users.