malteschmitz/WhileInterpreter

Exemplary parser and interpreter for simple while programs (literate programming)

Parser and Interpreter for While Programs

Exemplary Java implementation of a recursive decent parser, an abstract syntax tree (AST) data structure and a structural recursive interpreter over this data structure.

The purpose of this code is to demonstrate the concepts of syntax and semantics. For this purpose a little and very artificial but nevertheless Turing-complete programming language was defined.

The example code in the file mult.whl multiplies two integers using addition:

a := 2;
b := 4;
r := 0;
while (a) {
    r := r + b ;
    a := a - 1
}

Literate Programming Documentation

The code is documented in a literate programming style using Atlassian Docco

• Expression
  • Addition.java
  • Expression.java
  • Identifier.java
  • Int.java
  • Subtraction.java
• Interpreter
  • Evaluator.java
  • Interpreter.java
  • InterpreterException.java
  • Visitor.java
• Parser
  • Parser.java
  • SyntaxException.java
• Printer
  • ExpressionPrinter.java
  • ProgramPrinter.java
• Program
  • Assignment.java
  • Composition.java
  • Conditional.java
  • Loop.java
  • Program.java

Start with Parser.java and Interpreter.java where the formal syntax and semantics of while programs are defined.

Building

The maven project can be build with

mvn compile

which compiles the Java code and automatically generates the docco documentation in target/docco.

Without maven you can compile the Java code manually as follows:

mkdir -p target/classes
javac -d target/classes src/main/java/*.java src/main/java/**/*.java

Running

You can run the example code in mult.whl with

java -cp target/classes Main mult.whl

The expected output is

{a=0, b=4, r=8}

Exercise

In order to understand how this little application works, I suggest trying to extend it. For example you could try to implement the addition assignment += statement which assigns a variable to its current value plus the evaluation of the expression on the right hand side. This has to be done in three steps

1. Add a class AdditionAssignment in the program package with the attribute identifier of type Identifier storing the name of the variable that will be assigned and the attributed expression of type Expression storing the right hand side expression.

2. Update the Parser by extending the Stmt rule

Stmt = ... | Id "+=" Expr

3. Update the Interpreter by extending the sem function either by rewriting the new addition assignment statement as an assignment and an addition

sem(x "+=" e, v) = sem(x ":=" x "+" e, v)

or by directly applying the assignment and the addition

sem(x "+=" e, v) = v.update(x, v(x) + eval(e, v))