VIATRA2/Examples/VTCL/TypeChecking

Overview: Type checking

Type checking is introduced to the VTCL language in a backward compatible way. This means that any existing VTCL transformation should compile without modification. Moreover, it also implies that detailed type checking is only initiated if a type is defined explicitly for a variable (or an ASM function).

As the main extension of the language, one can add type information to

• all variables defined locally with the let rule
• a parameter of an ASM rule, graph pattern or graph transformation rule
• for the parameters and return values of ASM functions

Then the VIATRA2 parser checks if these variables are used in a type conformant way.

Typed variables within ASM rules

In ASM rules, types can be assigned to variables within the let rule with the following syntax VarName : TypeConst = InitValue. Here the type constant TypeConst should be a valid element of the model space, which can be resolved at compile time.

The following example demonstrates two typical type errors identified by the typechecker.

machine typechecking_errors {
  rule main() = 
  let X : datatypes.Integer = 0, 
      // Type error: Type incompatible initialization: expecting a term of type 'Integer' instead of 'String'	
      Y : datatypes.Integer = "oops0"
  in
    seq {
          // Type error: operation '*' undefined for types ('Integer', 'String') 
 	  update X = X * "oops1";
          // Type error: type incompatible assignment: left value is of type 'Integer', right value is of type 'String'
 	  update X = X + "oops2";
          // This is correct
 	  update X = X + 3;	  
    }
}

Typed variables in ASM functions

In case of ASM functions, types can be defined (separately) for each parameter (location) as well as for the return value with the following syntax:

asmfunction myFun (TypeConst1, TypeConst2) : ReturnTypeConst

machine typecheck_asmfun {
 
 asmfunction goals(datatypes.String) : datatypes.Integer  {
       // Correct initialization
 	("Messi") = 3;
       // Type error (left value): type incompatible initialization: expecting a term of type 'String' instead of type 'Integer'
       // Type error (right value): type incompatible initialization: expecting a term of type 'Integer' instead of type 'String'
 	(3) = "Giggs";
 }
 
 rule main () = let X : datatypes.Integer = 0, Y : datatypes.String = "Giggs" in seq {
      // Correct assignment
      update X = goals("Messi");
      // Type error: Type incompatible assignment: left value is of type 'String', right value is of type 'Integer'
      update Y = goals("Messi");
      // Correct assignment
      update goals(Y) = 2; 
      // Type error: Type incompatible initialization: expecting a term of type 'String' instead of type 'Integer'
      update goals(X) = 2; 	  
 }
}

Error messages

The following error messages are issued by the type checker. Note that currently, these messages are classified as warnings!

• Operator '-' is undefined for the argument type ('datatypes.String'): issued when checking an ASM term
• Operator '*' is undefined for the argument types ('datatypes.String', 'datatypes.String'): issued when checking an ASM term
• The type 'uml2.metamodel.Classifier' of actual parameter is not compatible with the type 'uml2.metamodel.Class' of symbolic parameter 'C'. See the definition of 'myPattern' : issued when calling a graph pattern using the find construct
• The type 'uml2.metamodel.Classifier' of variable reference is not compatible with the type 'uml2.metamodel.Class' of its definition.: issued in ASM rules or in the body of graph patterns
• Type incompatible assignment: left value is of type 'datatypes.String', right value is of type 'datatypes.Integer' : issued in case of an update rule
• Type incompatible initialization: expecting a term of type 'datatypes.String' instead of 'datatypes.Integer' : issued when initializing an ASM function

--Varro.mit.bme.hu 11:02, 25 May 2009 (UTC)