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javahaskelltypespolymorphismparametric-polymorphism
Konstantin Milyutin
Konstantin Milyutin

Reputation: 12356

Polymorphic values in Java

I'm studying Haskell and came across polymorphic values. These are the values that have different type depending on the context. For example, Nothing has type Maybe a or [] has type [a]. So [] is list of anything you want and you can use it everywhere any list is expected.

I struggle to find a similar thing in Java, apart from "billion-dollar mistake" null, which is basically of any type. May be generics with unbounded wildcard is similar, but again, I can't think of an example.

Is there something similar in Java?

Upvotes: 3

Views: 523

Answers (2)

chi
chi

Reputation: 116139

This is more a comment than an actual answer, but I require more space.

While the type systems of Java and Haskell are quite apart, some similarities can be found in certain features. Here are a few examples: just keep in mind that there is no perfect correspondence between the related constructs shown below.

Polymorphic functions (as in FP, not OOP polymorphism):

fst :: forall a b. (a, b) -> a
fst (x,y) = x

<A,B> A fst(A a, B b) {
    return a;
}

Note that polymorphic non-function values can not be translated as easily. Even in Haskell, values such as 3 :: Num a => a are functions in disguise. Further, [] :: [a] and Nothing :: Maybe a are also treated as functions, taking a "type" parameter in some stages of compilation -- albeit, IIRC at runtime these parameter disappear.

Type arguments:

data T a = T a Int a

class T<A> {
    A x;
    int n;
    A y;

    T(A x, int n, A y) {
        this.x = x; this.n = n; this.y = y;
    }
}

Algebraic data types (e.g. data with many constructors): these are not directly available in Java. Some other JVM languages, like Scala, have ADT supported directly in the language, as well as pattern patching and non-exhaustiveness warnings. In Java, you can use the so-called Visitor pattern to simulate ADT value elimination, which is a basic form of pattern matching.

Upvotes: 7

Tobias Brandt
Tobias Brandt

Reputation: 3413

A polymorphic value like [] can only be represented as a method in Java. In fact, this method already exists in the Arrays class and is called asList. And because of Java's type inference, you don't even need to provide type arguments:

import java.util.Arrays;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> myList = Arrays.asList();
    }
}

If you ignore all the noise, [] ≡ Arrays.asList().

Upvotes: 2

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