Type Equality and Compatibility

In an assignment and in passing an actual argument to a function, the types of the target and the expression being assigned or passed must be equal (with noted exceptions, such as assignment of nil to a reference type). When a function is defined, its type must be equal to the type of a function with the same name if one is in scope. Type equality is determined as follows.

• Two basic types are equal if and only if they are identical.
• Two tuple types are equal if and only if they are composed of equal types in the same order.
• Two array types are equal if and only if they are arrays of equal types. (The size of an array is not part of its type.)
• Two list types are equal if and only if they are composed from equal types.
• Two channel types are equal if and only if they transmit equal types.
• Two adt types are equal if and only if their data members are listed in the same order, have the same names, and have correspondingly equal types. The cyclic attribute of members is significant. Constant and function members of an adt do not enter into the comparison, nor does the name of the adt type itself. In particular, in the declarations:

		A: adt { x: ref B };

		B: adt { x: ref A };


 the types A and B are equal.

• Two ref adt types are equal if and only if they are references to equal adt types.
• Two module types are equal if and only if their data members are listed in the same order, and have the same names and the same types. Moreover, their functions (including functions within their adt members) have equal types and appear in the same order. However, the interleaving of function members among data members is insignificant. Constant members and type members do not enter into the comparison.
• Two function types are equal if their names are the same and if their return values have the same type and their argument lists have correspondingly equal types. The self attributes on arguments much match. Names given to arguments do no enter into the comparison.
• A type name represents the same type as the type from which it was constructed.
• When a module is loaded, the module stored in the file system must have a type that is compatible with the type mentioned in the load expression. A load type is compatible with a stored type if and only if the data members of the two types have the same names, the same types, and appear in the same order. Moreover, any adt or functions used by the program executing load must have a type equal to the corresponding members of the stored module. However, there are no restrictions on the ordering of these members, and the stored module can contain adt or function members not present in the module declaration present in the load program.