Write a clearly legible T to the left of each of the following that is true, and a clearly legible F to the left of each that is false.
In a typeless language, all type checking done at run time. T
Standard ML is a strongly typed language. Therefore, in Standard ML, it is not possible to get a type error at run time. T
Polymorphism is most useful in designing custom applications, and is rarely used in function libraries. F
Programs written in an object-oriented style tend to be organized significantly differently from programs written in a functional or procedural style. T
The class is an important concept of object-based programming. F
In most object-oriented languages, some type checking must be done at run time. T
Polymorphism without type variables typically requires that a programmer defeat the compile-time type system. Give an example where the type system would need to be defeated without type variables, but where type variables would allow the compiler to perform type checking.
Let f be the identity function, f(x) = x. Its type if
ANYTHING -> ANYTHING. If you say y = f(x), then the programmer
knows that y and x have the same type, but the compiler does not
know that. So the programmer must defeat the type system,
telling the compiler to treat y as having the same type as x.
For example, a program might say
int x;
...
y = (int)f(x);
telling the compiler information (that f(x) should have type int)
that it cannot glean from the type of f. Type variables
would allow the compiler to infer that y must have the same
type as x.
What is the most general polymorphic type of function f defined by f(x) = tail(right(x))? Use type variables. Function right takes the left-hand member of an ordered pair.
f: (A,[B]) -> [B]
where A and B are type variables.
What is the most general polymorphic type of the Astarte function filter? Use type variables. Filter is defined so that (filter f L) a list of all members x of list L such that f(x) is true. For example, if function positive returns true on a positive number and false on a nonpositive number, then (filter positive [9,-2,-3,6]) = [9,6].
filter: (A -> boolean) -> ([A] -> [A])
where A is a type variable.
In object-oriented programming, is a private data field of an object accessible only to that one object, or is it possible for other objects to access it directly?
In object-based programming, a private data field (variable) is accessible only to the object that contains it. In object-oriented programming, it is accessible to that object and to all other objects of the same class.
In a single-inheritance object-oriented language, is there a limit on the number of base classes that a class can have?
Yes. In a single-inheritance language, each class can have at most one base class.
In object-oriented programming, you imagine that objects carry functions with them. Yet, the functions are not really stored with the objects. How does an object locate its functions? How does it know which functions to select? What is the name of the system support that is responsible for locating functions?
An object carries a tag indicating its class. Functions are stored according to their class in a dispatch table. The part of the run-time support that locates functions in the dispatch table is called the dispatcher.
How does the mechanism for inheriting variables work in single-inheritance object-oriented languages? Is there a separate implementation of each selector for each class, or does one implementation of each selector work for all classes? How do the selector(s) work?
Variables are found by their position in the block of memory that represents an object's state. They are found at the same position regardless of the class of the object. So a single implementation works for all classes.
What is an abstract class?
An abstract class is a class with at least one virtual functions. You are not allowed to create an instance of an abstract class unless that instance is actually an instance of a subclass of the abstract class.
What are the characteristics of a virtual function? What makes it virtual?
A virtual function logically belongs to a class that is higher in the class hierarchy than the level at which function can be implemented. It is declared to belong to the higher level, but implemented at the lower levels.