Project 7: Alternate OO

In this project, you will complete an interpreter for JayMU. The JayMU system is strikingly different from the other languages in the Jay family in that we're writing a "wimpy interpreter"---it does not try to make a realistic model of memory, and it relies on the underlying Python interpreter's call stack to handle invocations. This allows us to concentrate on the parts that are new.

1. Set-up.

Copy the starter code from the course directory.

cp ~tvandrun/Public/cs365/proj7.tar .

You will complete the interpreterVisitor.py. This will involve not only finishing the InterpreterVisitor but also another visitor to modify the body of the method updates to replace enclosing scope variables with their values.

2. Tasks

You'll notice that interpreter has two instance variables: result, for the result of the most recently evaluated expression, and sigma, for the store. sigma is a dict associating variable names with values. Object values are dicts associating method names with method ASTs. Method ASTs include the return type and list of formals (see jaymu_ast.cfg) in addition to the body. The body is the only thing that gets updated in a method update.

Here is a brief description of what needs to be done for each visit method that needs to be finished.

• visit_CallStmt. There are many things that need to be taken care of for call statements:
  • Evaluate the actual parameters
  • Check to see if the method being called exists in the receiver, throwing a JayMuRuntimeError if not.
  • Save the old sigma and make a new one.
  • Set up the new sigma with the global variables, this, and formal parameters of the method, throwing a JayMuRuntimeError if there are too many or too few actual parameters.
  • Visit the method body.
  • Restore the old sigma when the method returns.
• visit_MethodUpdate. This method must check that the method being updated already exists in the "root" (the object whose method is being updated), raising a JayMuRuntimeError if it does not exist. Then it must make a copy of the replacement body (you may use the deepcopy() function from Python's copy library). Then it must take that new copy of the body and replace all the occurrences of enclosing scope variables with their current values. The easiest way to do this is to write a second visitor class (do it in the same file, interpreterVisitor.py) that will visit the body and modify it. (In the extra visitor that I wrote, there were only three methods: __init__(), visit_VariableWrapper(), and one other visit method.) The tricky part will be to make sure this extra visitor does not replace globals, formals and other locals of the method update body, or this. Also, the root will be a dict mapping method names to methods; you do not want to overwrite the entire method (signature and all) but only the body of that signature.
• visit_Instantiation. This must set self.result to a new dict that has associations for all the methods given. This will again require you to make a deep copy of the methods of the instnatiation and use the extra visitor you write earlier to replace all enclosing scope variables.

3. To turn in

To turn in this project, please copy your interpreterVisitor.py to

/cslab.all/linux/class/cs365/(your id)/proj7

I will grade your project by running it against a collection of test files.

Due: Fri, April 22, 5:00 pm. This will overlap with project 8.