Add conditional value emitting

Signed-off-by: Marcel Müller <neikos@neikos.email>

src/eval/mod.rs

@@ -93,15 +93,34 @@ pub fn execute(
         match instr {
             Instruction::NoOp => (),
             Instruction::AppendContent { content } => output.push_str(content),
-            Instruction::LoadFromContextToSlot { name, slot } => {
-                let value = scopes
-                    .get_scoped(name)
-                    .ok_or(EvaluationError::UnknownVariable(name.clone()))?;
+            Instruction::LoadFromContextToSlot {
+                name,
+                slot,
+                fail_on_not_found,
+            } => {
+                let value = scopes.get_scoped(name);
+
+                let value = if let Some(val) = value {
+                    val
+                } else {
+                    if *fail_on_not_found {
+                        return Err(EvaluationError::UnknownVariable(name.clone()));
+                    } else {
+                        &NomoValue::Undefined
+                    }
+                };
 
                 scopes.insert_into_slot(*slot, value.clone());
             }
             Instruction::EmitFromSlot { slot } => {
-                let value = scopes.get(slot).try_to_string().unwrap();
+                let value = scopes.get(slot);
+                let value = if let Some(value) = value.try_to_string() {
+                    value
+                } else if matches!(value, NomoValue::Undefined) {
+                    String::new()
+                } else {
+                    panic!("Unknown variable");
+                };
 
                 output.push_str(&value);
             }
@@ -286,4 +305,26 @@ mod tests {
         )
         "#);
     }
+
+    #[test]
+    fn check_conditional_access() {
+        let input = "Hello {{= unknown? }}";
+
+        let parsed = crate::lexer::parse(input.into()).unwrap();
+
+        let ast = crate::parser::parse(parsed.tokens()).unwrap();
+
+        let emit = crate::compiler::emit_machine(ast);
+
+        let context = Context::new();
+        let function_map = FunctionMap::default();
+
+        let output = execute(&function_map, &emit, &context);
+
+        insta::assert_debug_snapshot!(output, @r#"
+        Ok(
+            "Hello ",
+        )
+        "#);
+    }
 }