Equivalence

When making decisions about whether or not two types are compatible for assignment, Dana considers type equivalence at a structural level and ignores naming. The following two types are therefore compatible in Dana's type system:

data Person {
    char name[]
    int age
    }

data Address {
    char streetName[]
    int houseNumber
    }

Because there is no structural difference between these two types (they both have a character array field followed by an integer field) Dana considers them equivalent and compatible for inter-assignment. The same type compatibility rules apply to interface types.

Under these rules, note that the "packages" in which interfaces appear are treated as a programmer-level semantic annotation that is not relevant to the language in determining compatibility.

Inter-type assigns and hastype

Dana does not support an explicit cast operator. However, compatible types can be assigned as part of a regular assignment operation, for example:

    String a = new String("Hi")
    Data b = a
    String c = b

You can query at runtime whether or not a given type has a particular sub-type identity by using the hastype operator. As an example, consider a function that receives an Object instance as a parameter. We can then use hastype to determine whether or not this Object is structurally equivalent to a File type and then call File operations on it:

    void function(Object o)
        {
        if (o hastype File)
            {
            File f = o
            char buf[] = f.read(64)
            }
        }