Add read-only storage class for locals

source/ona/kym.zig

@@ -70,8 +70,18 @@ pub const RuntimeEnv = struct {
 		const OpcodeList = coral.list.Stack(Opcode);
 
 		const CompilationUnit = struct {
-			local_identifiers_buffer: [255][]const coral.io.Byte = [_][]const coral.io.Byte{""} ** 255,
+			locals_buffer: [255]Local = [_]Local{.{}} ** 255,
-			local_identifiers_count: u8 = 0,
+			locals_count: u8 = 0,
+
+			const ResolvedLocal = struct {
+				index: u8,
+				is_readonly: bool = false,
+			};
+
+			const Local =  struct {
+				identifier: []const coral.io.Byte = "",
+				is_readonly: bool = false,
+			};
 
 			fn compile_expression(self: *CompilationUnit, chunk: *Chunk, expression: ast.Expression) RuntimeError!void {
 				const number_format = coral.utf8.DecimalFormat{
@@ -204,20 +214,24 @@ pub const RuntimeEnv = struct {
 
 					.local_get => |local_get| {
 						try chunk.opcodes.push_one(.{
-							.push_local = self.resolve_local(local_get.identifier) orelse {
+							.push_local = (self.resolve_local(local_get.identifier) orelse {
 								return chunk.env.raise(error.OutOfMemory, "undefined local");
-							},
+							}).index,
 						});
 					},
 
 					.local_set => |local_set| {
 						try self.compile_expression(chunk, local_set.value_expression.*);
 
-						try chunk.opcodes.push_one(.{
+						const resolved_local = self.resolve_local(local_set.identifier) orelse {
-							.local_set = self.resolve_local(local_set.identifier) orelse {
 							return chunk.env.raise(error.BadSyntax, "undefined local");
-							},
+						};
-						});
+
+						if (resolved_local.is_readonly) {
+							return chunk.env.raise(error.BadSyntax, "cannot set a read-only declaration");
+						}
+
+						try chunk.opcodes.push_one(.{.local_set = resolved_local.index});
 					},
 
 					.field_get => |field_get| {
@@ -266,15 +280,23 @@ pub const RuntimeEnv = struct {
 						}
 					},
 
-					.local_var => |local_var| {
+					.declare => |declare| {
-						try self.compile_expression(chunk, local_var.assigned_expression);
+						if (self.resolve_local(declare.identifier) != null) {
+							return chunk.env.raise(error.BadSyntax, "declaration shadows existing one");
+						}
 
-						if (self.local_identifiers_count == self.local_identifiers_buffer.len) {
+						try self.compile_expression(chunk, declare.assigned_expression);
+
+						if (self.locals_count == self.locals_buffer.len) {
 							return chunk.env.raise(error.BadSyntax, "chunks may have a maximum of 255 locals");
 						}
 
-						self.local_identifiers_buffer[self.local_identifiers_count] = local_var.identifier;
+						self.locals_buffer[self.locals_count] = .{
-						self.local_identifiers_count += 1;
+							.identifier = declare.identifier,
+							.is_readonly = declare.storage != .variant,
+						};
+
+						self.locals_count += 1;
 					},
 
 					.block => |block| {
@@ -326,16 +348,21 @@ pub const RuntimeEnv = struct {
 				}
 			}
 
-			fn resolve_local(self: *CompilationUnit, local_identifier: []const coral.io.Byte) ?u8 {
+			fn resolve_local(self: *CompilationUnit, local_identifier: []const coral.io.Byte) ?ResolvedLocal {
-				if (self.local_identifiers_count == 0) {
+				if (self.locals_count == 0) {
 					return null;
 				}
 
-				var index = @as(u8, self.local_identifiers_count - 1);
+				var index = @as(u8, self.locals_count - 1);
 
 				while (true) : (index -= 1) {
-					if (coral.io.are_equal(local_identifier, self.local_identifiers_buffer[index])) {
+					const local = &self.locals_buffer[index];
-						return index;
+
+					if (coral.io.are_equal(local_identifier, local.identifier)) {
+						return .{
+							.index = index,
+							.is_readonly = local.is_readonly,
+						};
 					}
 
 					if (index == 0) {

source/ona/kym/ast.zig

@@ -129,6 +129,11 @@ pub const Expression = union (enum) {
 	});
 };
 
+pub const DeclarationStorage = enum {
+	variant,
+	readonly,
+};
+
 const ExpressionBuilder = fn (self: *Tree) ParseError!Expression;
 
 pub const ParseError = error {
@@ -139,7 +144,8 @@ pub const ParseError = error {
 pub const Statement = union (enum) {
 	@"return": ?Expression,
 
-	local_var: struct {
+	declare: struct {
+		storage: DeclarationStorage,
 		identifier: []const coral.io.Byte,
 		assigned_expression: Expression,
 	},
@@ -672,8 +678,34 @@ pub const Tree = struct {
 				self.tokenizer.skip_newlines();
 
 				return .{
-					.local_var = .{
+					.declare = .{
 						.assigned_expression = try self.parse_expression(),
+						.storage = .variant,
+						.identifier = identifier,
+					},
+				};
+			},
+
+			.keyword_let => {
+				self.tokenizer.skip_newlines();
+
+				const identifier = switch (self.tokenizer.token) {
+					.identifier => |identifier| identifier,
+					else => return self.report("expected identifier after `let` declaration statement"),
+				};
+
+				self.tokenizer.skip_newlines();
+
+				if (self.tokenizer.token != .symbol_equals) {
+					return self.report("expected `=` after declaration identifier");
+				}
+
+				self.tokenizer.skip_newlines();
+
+				return .{
+					.declare = .{
+						.assigned_expression = try self.parse_expression(),
+						.storage = .readonly,
 						.identifier = identifier,
 					},
 				};

source/ona/kym/tokens.zig

@@ -45,6 +45,7 @@ pub const Token = union(enum) {
 	keyword_else,
 	keyword_elif,
 	keyword_var,
+	keyword_let,
 
 	pub fn text(self: Token) []const coral.io.Byte {
 		return switch (self) {
@@ -93,6 +94,7 @@ pub const Token = union(enum) {
 			.keyword_elif => "elif",
 			.keyword_else => "else",
 			.keyword_var => "var",
+			.keyword_let => "let",
 		};
 	}
 };
@@ -231,6 +233,14 @@ pub const Tokenizer = struct {
 							}
 						},
 
+						'l' => {
+							if (coral.io.ends_with(identifier, "et")) {
+								self.token = .keyword_let;
+
+								return;
+							}
+						},
+
 						'n' => {
 							if (coral.io.ends_with(identifier, "il")) {
 								self.token = .keyword_nil;