UI configuration via Lua scripts

[joelim-work]

So today I decided to bite the bullet and experiment with Lua scripting using the GopherLua library. It turns out to be straightforward enough to get a PoC working, in this case customizing the prompt line:

Diff (click to expand):

diff --git a/go.mod b/go.mod
index 7e7f174..9082ccb 100644
--- a/go.mod
+++ b/go.mod
@@ -7,6 +7,7 @@ require (
 	github.com/fsnotify/fsnotify v1.9.0
 	github.com/gdamore/tcell/v3 v3.1.2
 	github.com/rivo/uniseg v0.4.7
+	github.com/yuin/gopher-lua v1.1.2
 	golang.org/x/sys v0.43.0
 	golang.org/x/term v0.42.0
 )
diff --git a/go.sum b/go.sum
index b1164b5..affd22f 100644
--- a/go.sum
+++ b/go.sum
@@ -11,6 +11,8 @@ github.com/lucasb-eyer/go-colorful v1.3.0/go.mod h1:R4dSotOR9KMtayYi1e77YzuveK+i
 github.com/rivo/uniseg v0.4.7 h1:WUdvkW8uEhrYfLC4ZzdpI2ztxP1I582+49Oc5Mq64VQ=
 github.com/rivo/uniseg v0.4.7/go.mod h1:FN3SvrM+Zdj16jyLfmOkMNblXMcoc8DfTHruCPUcx88=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+github.com/yuin/gopher-lua v1.1.2 h1:yF/FjE3hD65tBbt0VXLE13HWS9h34fdzJmrWRXwobGA=
+github.com/yuin/gopher-lua v1.1.2/go.mod h1:7aRmXIWl37SqRf0koeyylBEzJ+aPt8A+mmkQ4f1ntR8=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
diff --git a/os.go b/os.go
index 791a83c..2adf0bf 100644
--- a/os.go
+++ b/os.go
@@ -37,6 +37,7 @@ var (
 	gConfigPaths []string
 	gColorsPaths []string
 	gIconsPaths  []string
+	gLuaPaths    []string
 	gFilesPath   string
 	gMarksPath   string
 	gTagsPath    string
@@ -104,6 +105,11 @@ func init() {
 		filepath.Join(config, "lf", "icons"),
 	}
 
+	gLuaPaths = []string{
+		filepath.Join("/etc", "lf", "init.lua"),
+		filepath.Join(config, "lf", "init.lua"),
+	}
+
 	data := cmp.Or(
 		os.Getenv("LF_DATA_HOME"),
 		os.Getenv("XDG_DATA_HOME"),
diff --git a/os_windows.go b/os_windows.go
index 5ed13d0..58b379b 100644
--- a/os_windows.go
+++ b/os_windows.go
@@ -34,6 +34,7 @@ var (
 	gConfigPaths []string
 	gColorsPaths []string
 	gIconsPaths  []string
+	gLuaPaths    []string
 	gFilesPath   string
 	gTagsPath    string
 	gMarksPath   string
@@ -93,6 +94,11 @@ func init() {
 		filepath.Join(config, "lf", "icons"),
 	}
 
+	gLuaPaths = []string{
+		filepath.Join(os.Getenv("ProgramData"), "lf", "init.lua"),
+		filepath.Join(config, "lf", "init.lua"),
+	}
+
 	data := cmp.Or(
 		os.Getenv("LF_DATA_HOME"),
 		os.Getenv("XDG_DATA_HOME"),
diff --git a/ui.go b/ui.go
index 5c3d48a..d6eb360 100644
--- a/ui.go
+++ b/ui.go
@@ -19,6 +19,7 @@ import (
 
 	"github.com/gdamore/tcell/v3"
 	"github.com/rivo/uniseg"
+	"github.com/yuin/gopher-lua"
 	"golang.org/x/term"
 )
 
@@ -540,6 +541,7 @@ type ui struct {
 	rulerErr    error              // `rulerfile` parse error (if any)
 	currentFile string             // last path passed to `on-select`
 	pasteEvent  bool               // whether paste event is active (to ignore pasted input in Normal mode)
+	lua         *lua.LState
 }
 
 func newUI(screen tcell.Screen) *ui {
@@ -560,6 +562,13 @@ func newUI(screen tcell.Screen) *ui {
 	}
 	ui.ruler, ui.rulerErr = parseRuler(gOpts.rulerfile)
 
+	ui.lua = lua.NewState()
+	for _, path := range gLuaPaths {
+		if _, err := os.Stat(path); !os.IsNotExist(err) {
+			ui.lua.DoFile(path)
+		}
+	}
+
 	return ui
 }
 
@@ -699,7 +708,16 @@ func (ui *ui) drawPromptLine(nav *nav) {
 	}
 	prompt = strings.ReplaceAll(prompt, "%S", "")
 
-	ui.promptWin.print(ui.screen, 0, 0, st, prompt)
+	if err := ui.lua.CallByParam(lua.P{
+		Fn:      ui.lua.GetGlobal("prompt"),
+		NRet:    1,
+		Protect: true,
+	}, lua.LString(pwd), lua.LString(fname)); err == nil {
+		result := ui.lua.Get(-1).String()
+		ui.promptWin.print(ui.screen, 0, 0, tcell.StyleDefault, result)
+	} else {
+		ui.promptWin.print(ui.screen, 0, 0, st, prompt)
+	}
 }
 
 // Deprecated: Only called by drawRuler, which will eventually be replaced by drawRulerFile

Lua script ~/.config/lf/init.lua:

function prompt(pwd, fname)
    return string.format("\27[31m%s\27[32m%s\27[0m", pwd, fname)
end

The current PoC achieves this by doing the following:

1. Create a Lua VM (currently in the ui struct since this is a PoC, but could be moved to app)
2. Set gLuaPaths which contains the paths to lookup Lua scripts (called init.lua)
3. Loop through gLuaPaths and execute each init.lua script (in this case define the prompt function)
4. When drawing the prompt line, invoke the prompt function, passing in relevant information, and display the result

---

This does open up a whole new range of possibilities, for example:

• Virtually anything in the UI could be customized so long as the relevant information can be passed in
• UI configuration options (especially those ending in fmt), the colors/icons files and the recently introduced ruler file would be rendered completely obsolete
• Lua scripting could also be used to script custom commands such as :toggle; down
• In fact the entire lfrc file could be made obsolete as it is just a runtime script evaluated upon startup and could be replaced by a Lua script

I doubt I'll have the time to look at this in the near future, but anyway I'll leave this here in case anyone is interested in working on this further.