A lightweight, self-contained expression parser & evaluator implemented in pure Java using functional parser-combinator techniques.

• Parser-Combinator Core – parser.* provides Parser<T>, combinators (map, filter, zip, optional, zeroOrMore, …) and utility parsers.
• Arithmetic Grammar – grammar.ExpressionGrammar recognises integers, floats, addition, subtraction, multiplication, and division with correct precedence.
• Modern AST – Built with Java 17 sealed interfaces & records (ast.AST) for strong exhaustiveness checks.
• Evaluation Engine – eval.Eval traverses the AST and produces Long results (multiplication/division coming soon).
• Zero Dependencies – Compiles and runs with nothing but a JDK 24+.
• Self-Contained Tests – ExprEvaluator.main() performs assertion-based smoke tests and prints All Test Successful on success.

└── src ├── ast # Sealed interfaces & records representing the syntax tree ├── eval # AST Evaluation Logic ├── ExprEvaluator.java ├── grammar # Grammar built via Parser combinator ├── model ├── parser # Fully Functional Parser Library from Scratch ├── repl └── test 

• Java 24 or newer (for records, sealed interfaces, and pattern matching).

• -1 + ((1 - 2) * 3) + 4.0 / 2 => -2.0D

• Running the tests

 java -ea src/ExprEvaluator.java

• Running REPL

 java src/ExprEvaluator.java -r # or directly ./src/ExprEvaluator.java -r

1. Parser-Combinator Library The generic interface Parser<T> exposes parse(String) returning an Optional<ParseResult<T>>. Core combinators (map, filter, zeroOrMore, oneOrMore, etc.) allow composition of complex parsers in a functional style.

2. Grammar Construction (ExpressionGrammar)

   number → integer | float primary → number | "(" expr ")" factor → "+" factor | "-" factor | primary term → factor (('*'|'/') factor)* expr → term (('+'|'-') term)* 

   The grammar is encoded declaratively by composing smaller parsers with combinators like zip, optional, and oneOf.

3. AST Representation (ast.AST) Uses sealed interfaces (AST.Num) and records (Expr, Term, Factor) ensuring exhaustiveness in switch statements.

4. Evaluation (eval.Eval) Traverses the AST to compute results. Addition/Subtraction for integers is implemented; multiplication/division and floating-point support are planned.

• Implement MULT / DIV evaluation and float maths
• Add parentheses and unary operator support
• Provide interactive REPL
• Handle Spaces

Inspired by functional parser-combinator libraries in Haskell & Scala.