Add example with lua built-in function

examples/03-UseGoFuncInLua/main.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"doll/internal/log"
+	"os"
+
+	lua "github.com/yuin/gopher-lua"
+)
+
+// square multiplies number by itself.
+func square(L *lua.LState) int {
+	// get first (1) function argument and convert to int
+	i := L.ToInt(1)
+
+	// make calculation and cast to LNumber
+	ln := lua.LNumber(i * i)
+
+	// Push it to the stack
+	L.Push(ln)
+
+	// Notify that we pushed one value to the stack
+	return 1
+}
+
+func main() {
+	// Init log subsystem.
+	log.Init("info", os.Stderr)
+
+	file := "example_data/03-test.lua"
+
+	// Load the Lua script file.
+	log.Infof("Loading %s", file)
+
+	// We can execute script directly via L.DoFile("./example_data/03-test.lua") but in practice we'll get scripts as
+	// strings - snippets from templates - so, testing/checking how it works is not use for project.
+	luaCode, err := os.ReadFile(file)
+
+	if err != nil {
+		log.Fatalf("Error reading Lua file %s: %s", file, err)
+	}
+
+	log.Info("Creating new Lua state")
+	L := lua.NewState()
+	defer L.Close()
+
+	log.Info("Registering go function as lua built-in function")
+	L.SetGlobal("square", L.NewFunction(square))
+
+	log.Info("Evaluating Lua code")
+	if err := L.DoString(string(luaCode)); err != nil {
+		log.Errorf("Error evaluating Lua code: %s", err)
+
+		os.Exit(1)
+	}
+}
+
+/* vim: set ft=go noet ai ts=4 sw=4 sts=4: */