Robot-Control DSL

What is this

A fully self-contained domain-specific language for commanding a simulated robot in a two-dimensional grid world. Defined entirely in a single JavaCC grammar (Robot.jj), it supports variable definitions, first-class functions, conditionals, loops, and direct invocation of robot-world APIs without generating an intermediate AST.

How it works

1. Lexical and Syntactic Specification

   • JavaCC grammar: uses LOOKAHEAD=1 and IGNORE_CASE=true to implement an LL(1) parser.
   • Token declarations: keywords (defvar, defun, if, loop, repeat, not), punctuation ((, ), :), identifiers (NAME), numeric literals (NUMBER), constants (CONS), and predicate forms (facing?, blocked?, etc.) are specified in the TOKEN section.
   • Skip rules: whitespace and line breaks are discarded via SKIP.

2. Integrated Parsing and Execution

   • Recursive-descent productions: each grammar production is annotated with Java code that either records tokens into a buffer (codeInDefinition) or issues side-effecting calls to RobotWorldDec when not in definition mode.
   • No separate AST: function definitions accumulate their source text in codeInDefinition and, upon completion, store a FunctionValue (parameters + code string) in the current Scope. Invocation re-parses that stored code fragment via a fresh parser instance.

3. Scope and Static Analysis

   • Stack<Scope>: every block, function definition, or conditional pushes a new Scope object.
   • Variable resolution: Scope maps names to their string-encoded values ("NUMBER 5", "CONS dim", "KEYWORD north", etc.). On each reference, the parser enforces that the identifier is defined in the current or any enclosing scope.
   • Function signatures: stored as keys of the form "fname arity" mapping to FunctionValue. Redeclaration or arity mismatches produce ParseException.

4. Control Structures without AST

   • if / loop / repeat: each evaluates a boolean condition by invoking COND(). If outside definition context, the parser toggles a canExecute flag and either executes or skips the nested productions.
   • Dynamic re-entry: loops and recursion re-initialize parser state on stored code strings, enabling repeated execution without an explicit AST or bytecode.

5. Command Semantics

   • Movement and rotation: high-level commands compute relative facing changes (front, back, left, right) and dispatch to world.moveForward and world.turnRight.
   • Interaction predicates: methods like facingNorth(), isBlocked(), canMove() query the RobotWorldDec API for grid boundaries, obstacles, and resource availability.

All analysis (lexical, syntactic, and static-scope checks) and execution logic reside in Robot.jj, leveraging JavaCC’s generated parser framework for a compact yet powerful embedded DSL.