Add default error callback and callbacks example

examples/callbacks.luau

@@ -0,0 +1,4 @@
+--!nocheck
+--!nolint UnknownGlobal
+
+error("Oh no! Something went very very wrong!")

examples/callbacks.rs

@@ -0,0 +1,24 @@
+use smol_mlua::{
+    mlua::prelude::{Lua, LuaResult},
+    Callbacks, Runtime,
+};
+
+const MAIN_SCRIPT: &str = include_str!("./callbacks.luau");
+
+pub fn main() -> LuaResult<()> {
+    // Set up persistent lua environment
+    let lua = Lua::new();
+
+    // Load the main script into a runtime
+    let rt = Runtime::new(&lua)?;
+    let main = lua.load(MAIN_SCRIPT);
+
+    // Inject default value & error callbacks - this will print lua errors to stderr
+    Callbacks::default().inject(&lua);
+
+    // Run the main script until completion
+    rt.push_main(&lua, main, ());
+    rt.run_blocking(&lua);
+
+    Ok(())
+}

lib/callbacks.rs

@@ -6,7 +6,6 @@ type ErrorCallback = Box<dyn for<'lua> Fn(&'lua Lua, LuaThread<'lua>, LuaError)
 const FORWARD_VALUE_KEY: &str = "__runtime__forwardValue";
 const FORWARD_ERROR_KEY: &str = "__runtime__forwardError";
 
-#[derive(Default)]
 pub struct Callbacks {
     on_value: Option<ValueCallback>,
     on_error: Option<ErrorCallback>,
@@ -33,6 +32,16 @@ impl Callbacks {
         self
     }
 
+    pub fn without_value_callback(mut self) -> Self {
+        self.on_value.take();
+        self
+    }
+
+    pub fn without_error_callback(mut self) -> Self {
+        self.on_error.take();
+        self
+    }
+
     pub fn inject(self, lua: &Lua) {
         // Create functions to forward values & errors
         if let Some(f) = self.on_value {
@@ -72,3 +81,17 @@ impl Callbacks {
         }
     }
 }
+
+impl Default for Callbacks {
+    fn default() -> Self {
+        Callbacks {
+            on_value: Some(Box::new(default_value_callback)),
+            on_error: Some(Box::new(default_error_callback)),
+        }
+    }
+}
+
+fn default_value_callback(_: &Lua, _: LuaThread, _: LuaValue) {}
+fn default_error_callback(_: &Lua, _: LuaThread, e: LuaError) {
+    eprintln!("{e}");
+}