1. Overview

MuLang is a small, embeddable language for validating and executing expressions and imperative programs supplied as text.

1.1. Design goals

The language is designed to:

• support expression-only and statement-based compilation;
• use a small, explicitly defined type system;
• obtain global variables, functions, and host structured types from a host-provided environment;
• declare structural object types in source programs;
• prevent implicit access to host-specific operations;
• produce deterministic diagnostics before execution whenever possible;
• preserve the same language semantics across conforming implementations;
• allow controlled mutation of host-supplied objects and arrays;
• remain suitable for execution with cancellation and resource limits.

This specification defines MuLang without requiring knowledge of another language or platform.

1.2. Non-goals

MuLang does not provide:

• classes, inheritance, interfaces, or generics;
• function values;
• asynchronous functions;
• function overloads;
• optional or variadic function parameters;
• tuples;
• heterogeneous arrays;
• general union types;
• exception handling in source code;
• reflection or implicit access to host members;
• a privileged or implicitly imported standard library;
• comments;
• flow-sensitive type narrowing;
• assignment expressions.

1.3. Compilation modes

Every compilation MUST select expression mode or program mode.

1.3.1. Expression mode

Expression mode accepts exactly one expression followed by the end of the source text.

The expression value is the result of execution. A void-returning function call is not valid as the root expression.

Statements, including return, are not valid in expression mode.

For example, this expression produces the int value 7:

1 + 2 * 3

1.3.2. Program mode

Program mode accepts zero or more top-level type declarations, zero or more function declarations, zero or more executable statements, and the end of the source text, in that order. A type declaration MUST NOT follow a function or executable statement. A function declaration MUST NOT follow an executable top-level statement.

User-defined object types are specified in Section 4.5. User-defined functions are specified in Section 7. Their availability depends on the language profile.

A pure expression is not a statement. A function call is the only expression permitted as an expression statement.

A non-void program MUST return a value on every reachable path. A void program MAY complete without an explicit return.

For example, this non-void program produces the int value 7:

var value = 1 + 2 * 3;
return value;

The following is invalid because the pure expression is not a statement:

1 + 2;
return 3;