Add defaults for Value

src/lib.rs

@@ -22,34 +22,55 @@ use internal::{
 
 /// The primary trait exposed by this crate. Implement this trait if you would
 /// like to be able to render a type in a template.
+///
+/// NOTE: There are defaults implemented for every method on this trait:
+/// - print: prints nothing
+/// - lookup: always returns None
+/// - has: always returns None
+/// - is_truthy: always returns true
+/// - index: always returns None
+///
+/// You only need to implement the methods whose behaviors you want to change
 pub trait Value: core::fmt::Debug {
 	/// Print this value to the given string.
 	///
 	/// See also [`internal::parse::Print`] and [`internal::vm::ops::Op::ValuePrint`]
-	fn print(&self, out: &mut String);
+	fn print(&self, out: &mut String) {
+		let _ = out;
+	}
 
 	/// Lookup a field name in this value. The returned [`Value`] will be set as
 	/// the current value inside the block.
 	///
 	/// See also [internal::vm::ops::Op::ValueLookup]
-	fn lookup(&self, name: &str) -> Option<&dyn Value>;
+	fn lookup(&self, name: &str) -> Option<&dyn Value> {
+		let _ = name;
+		None
+	}
 
 	/// Like [`Option::map`], but for a value. The returned [`Value`] will be set
 	/// as the current value inside the block.
 	///
 	/// See also [`internal::parse::Has`] and [`internal::vm::ops::Op::ValueHas`]
-	fn has(&self) -> Option<&dyn Value>;
+	fn has(&self) -> Option<&dyn Value> {
+		None
+	}
 
 	/// Check if a value is truthy.
 	///
 	/// See also [`internal::parse::If`] and [`internal::vm::ops::Op::ValueTruthy`]
-	fn is_truthy(&self) -> bool;
+	fn is_truthy(&self) -> bool {
+		true
+	}
 
 	/// Lookup the current index in this value. The returned [`Value`] will be
 	/// set as the current value inside the block.
 	///
 	/// See also [`internal::parse::For`] and [`internal::vm::ops::Op::ValueIndex`]
-	fn index(&self, index: usize) -> Option<&dyn Value>;
+	fn index(&self, index: usize) -> Option<&dyn Value> {
+		let _ = index;
+		None
+	}
 }
 
 macro_rules! value_for_int {