Missing variable error implementation

src/errors.cc

@@ -134,6 +134,7 @@ void unreachable(Location loc)
 std::ostream& operator<<(std::ostream& os, Error const& err)
 {
 	std::string_view short_description = visit(Overloaded {
+		[](errors::Missing_Variable const&)                     { return "Cannot find variable"; },
 		[](errors::Failed_Numeric_Parsing const&)               { return "Failed to parse a number"; },
 		[](errors::Not_Callable const&)                         { return "Value not callable"; },
 		[](errors::Undefined_Operator const&)                   { return "Undefined operator"; },
@@ -154,6 +155,17 @@ std::ostream& operator<<(std::ostream& os, Error const& err)
 
 	auto const loc = err.location;
 	visit(Overloaded {
+		[&](errors::Missing_Variable const& err) {
+			os << "I encountered '" << err.name << "' that looks like variable but\n";
+			os << "I can't find it in surrounding scope or in one of parent's scopes\n";
+			os << "\n";
+
+			Lines::the.print(os, std::string(loc->filename), loc->line, loc->line);
+
+			os << "\n";
+			os << "Variables can only be references in scope (block) where they been created\n";
+			os << "or from parent blocks to variable block\n";
+		},
 		[&](errors::Unrecognized_Character const& err) {
 			os << "I encountered character in the source code that was not supposed to be here.\n";
 			os << "  Character Unicode code: U+" << std::hex << err.invalid_character << '\n';

src/interpreter.cc

@@ -749,8 +749,15 @@ Result<Value> Interpreter::eval(Ast &&ast)
 					return Value::from(std::move(ast.token.source).substr(1));
 				}
 
-				auto const value = env->find(std::string(ast.token.source));
-				assert(value, "Missing variable error is not implemented yet: variable: "s + std::string(ast.token.source));
+				auto name = std::string(ast.token.source);
+
+				auto const value = env->find(name);
+				if (!value) {
+					return Error {
+						.details  = errors::Missing_Variable { .name = std::move(name) },
+						.location = ast.location
+					};
+				}
 				return *value;
 			}
 			return Value{};

src/musique.hh

@@ -104,6 +104,13 @@ namespace errors
 		std::vector<std::string> possibilities;
 	};
 
+	/// When user tries to use variable that has not been defined yet.
+	struct Missing_Variable
+	{
+		/// Name of variable
+		std::string name;
+	};
+
 	/// Collection of messages that are considered internal and should not be printed to the end user.
 	namespace internal
 	{
@@ -126,6 +133,7 @@ namespace errors
 		Expected_Expression_Separator_Before,
 		Failed_Numeric_Parsing,
 		Literal_As_Identifier,
+		Missing_Variable,
 		Not_Callable,
 		Undefined_Operator,
 		Unexpected_Empty_Source,