diff --git a/README.md b/README.md new file mode 100644 index 0000000..5a0557f --- /dev/null +++ b/README.md @@ -0,0 +1,78 @@ +# 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](https://bitbucket.org/doklovic_atlassian/atlassian-docco) + +Have a look a the [Parser](https://malteschmitz.github.io/WhileInterpreter/docco/vertical/src/main/java/parser/Parser.java.html) and the [Interpreter](https://malteschmitz.github.io/WhileInterpreter/docco/vertical/src/main/java/interpreter/Interpreter.java.html) where the formal syntax and semantics are defined, too. + +## 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 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 is. For example you could try to implement the `++` operator which increments a variable. This has to be done in three steps + +1. Add a class `Increment` in the `program` package with the attribute `identifier` of type `Identifier` storing the name of the variable that will be incremented. + +2. Update the `Parser` by extending the `Stmt` rule + + ``` + Stmt = ... | Id "++" + ``` + +3. Update the `Interpreter` by extending the `sem` function either by rewriting the increment operator as an assignment + + ``` + sem(x "++", v) = sem(x ":=" x "+" 1, v) + ``` + + or by directly applying the incrementation + + ``` + sem(x "++", v) = v.update(x, v(x) + 1) + ```