Can I avoid eager ambiguity resolution for trait implementations with generics? No. Is it possible to have [ambiguity resolution to be done at the call site, rather than at the definition site]? No. There’s a (long-delayed) RFC for specialization that will allow overlapping trait implementations, but only when one of them is more specific than … Read more
UPDATE: This question was the basis of my blog article on the 3rd of February 2011. Thanks for the great question! This is legal, it is not circular, and it is fairly common. I personally dislike it. The reasons I dislike it are: It is excessively clever; as you’ve discovered, clever code is hard for … Read more
This is indeed correct behaviour as overload resolution takes place at compile time (it would be a pretty expensive operation to take place at runtime). Therefore from within test(value:), the only thing the compiler knows about value is that it’s of some type that conforms to DispatchType – thus the only overload it can dispatch … Read more
Generics in Java doesn’t allow creation of arrays with generic types. You can cast your array to a generic type, but this will generate an unchecked conversion warning: public class HashTable<K, V> { private LinkedList<V>[] m_storage; public HashTable(int initialSize) { m_storage = (LinkedList<V>[]) new LinkedList[initialSize]; } } Here is a good explanation, without getting into … Read more
Protocols with type aliases cannot be used this way. Swift doesn’t have a way to talk directly about meta-types like ValidationRule or Array. You can only deal with instantiations like ValidationRule where… or Array<String>. With typealiases, there’s no way to get there directly. So we have to get there indirectly with type erasure. Swift has … Read more
Add the method to the interface constraint, without forgoing generics: type AB interface { *A | *B some() bool } func some[T AB](x T) bool { return x.some() // works } This restricts T to types that are either *A or *B and declare some() bool method. However, as you already found out, this is … Read more
This is F#’s embarrassing skeleton in the closet. Try this: > let mapPair f (x,y) = (f x, f y) val mapPair : f:(‘a -> ‘b) -> x:’a * y:’a -> ‘b * ‘b Fully generic! Clearly, function application and tuples work. Now try this: > let makeList a b = [a;b] val makeList : … Read more
You’re probably going to need to take a look at using wildcard types for generics. Here’s a quick link: What is PECS (Producer Extends Consumer Super)? Quick answer: change the type to List<? extends AbstractItem> Why can’t you just assign this? Imagine the code here… List<AbstractItem> foo = new ArrayList<SharpItem>(); foo.add(new BluntItem()); The static typing … Read more
Basically you have to add one more type parameter to the constraint to make T convertible to its pointer type. In its most basic form, this technique looks like the following (with an anonymous constraint): func Foo[T any, PT interface { *T; M() }]() { p := PT(new(T)) p.M() // calling method on non-nil pointer … Read more
Though maps and slices in go are generic themselves, they are not covariant (nor could they be, since interfaces aren’t generics). It’s part of working with a language that doesn’t have generics, you will have to repeat some things. If you really just need to get the keys of any old map, you can use … Read more