Makevisitors: Pass 2

Generate the Java visitor interfaces

If a Haskell data type is not simple, then we will call it complex. A complex Haskell data type has at least one tag that has associated values. For a complex data type, we will generate a class hierarchy and a visitor interface. In this pass, we’ll focus on the visitor interface.

Here is an example of a complex Haskell data type, and its corresponding abstract syntax:

    -- concrete Haskell declaration
    data Expr = Value {value :: Integer}
              | BinOp {operator :: Op, leftOperand :: Expr, rightOperand :: Expr}

    -- abstract syntax (an instance of TyDefs)
    [Datatype "Expr" [  Record "Value" [  ("value", Base "Integer")  ]
                      , Record "BinOp" [  ("operator",     Base "Op")
                                        , ("leftOperand",  Base "Expr")
                                        , ("rightOperand", Base "Expr")  ]]

And here is a Java visitor for this datatype, along with its corresponding abstract syntax:

    // concrete Java declaration
    public interface ExprVisitor {
        public void visitValue(Value valueVal);    
        public void visitBinOp(BinOp binOpVal);
    }

    -- abstract syntax (an instance of JDeclaration)
    InterfaceDef "ExprVisitor" [] 
        [  MethodDef [Public] (Typename "void") "visitValue" [(Typename "Value","valueVal")] NoBody
         , MethodDef [Public] (Typename "void") "visitBinOp" [(Typename "BinOp","binOpVal")] NoBody ]

Your task: In Translate.hs, complete the implementation of the second pass.

After you complete this pass, you should be able to generate Visitor classes for any complex data type. The Java code likely won’t compile because you haven’t yet implemented the part that generates classes for that type.

Testing. It is largely on you to test your code. We suggest:

• In makevisitors.hs, comment out the calls for the unimplemented passes (just make sure to uncomment them as you implement them):
  • genAbstractSuperclasses
  • genConcreteSubclasses
• Write a sample file that contains only complex data types and check that the generated visitors matches your expectations.
• Run make test, which will generate code for lots of different examples. Check that the visitors are generated as you expected. You can compare your code against the code generated by the reference implementation, specifically:
  • exprTest/ExprVisitor.java
  • jsonTest/JValueVisitor.java
  • lambdaTest/ExprVisitor.java
  • lambdaTest/ValueVisitor.java
  • randomTest/FobleVisitor.java
  • randomTest/WidgetVisitor.java
  • stack/StackInstrVisitor.java

  These files should match yours. (The other files were generated by a complete implementation, whereas you currently have a partial implementation.)