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When making decisions about whether or not two types are compatible for assignment, Dana considers type equivalence at a structural level and ignores semantics. 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. This leaves the programmer free to decide what semantic type equivalence (for example the names of types) means in their context.

Under these rules, note that the “packages” in which interfaces appear are purely a programmer-level semantic annotation that is not relevant to the language in determining compatibility.Inter-type assigns and hastype

As with other modern systems programming languages, 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)
        }
    }