I want to start with what I know, or at least I think I know, so what I'm asking would be more clear.
First of all, I know that you can declare a variable of a supertype and assign an object of a subtype to take advantage of polymorphism with Inheritence and Interfaces.
I know that generics provide type safety because the type parameters are invariant by definition, so where A is a subtype of B, Foo<A> is not necessarily a subtype of Foo<B>, and may not be used in place depending on mutability of the object. With this, possible exceptions that could arise at runtime due to dynamic dispatching can be caught in compile time.
They also help to define a generic logic for different types: Like in Lists where you have collections of type A objects, but it doesn't change the implementation for type B objects.
Also, I understood why MutableList<String> doesn't count as the subtype of MutableList<Any> because that could result in cases where you create a variable with type MutableList<Any> that holds a reference to a MutableList<String> object, and add an Int element to a List of Strings, which is obviously a problem.
I also understood why List version of the previous example works because Lists are immutable so you can't make any modification to the object that could result in type mismatches.
Lastly, I know that type parameters with in can only be used as function parameters, being consumed, and the ones with out can be used as the function return types, being produced.
Now to the part what I don't understand:
I didn't quite understand what the words consumer and producer actually means in terms of in and out. What does it mean for a type to be in consumed or produced position? Does that mean the object with that type can only be read or write only? Does that have anything to do with the object at all?
What would be the behaviour of the object if, let's say, we don't define it using in or out, or, opposite, we define it using in or out, not talking about the subtype-supertype relationship that I explained above.
I spend the last few days looking at different explanations of this, but I found the lack of examples a big problem, especially because that's how I usually learn.
I can use these concepts in code, but the lack of underlying knowledge or the logic greatly disturbs me, so please, if you decide to take the time to write an explanation, provide it with examples and counter examples of why or how a certain idea works.
Just one correction to your first bullet points: List is not immutable; it is read-only. A List could be an up-cast mutable implementation and some other object that references it could be mutating it.
Producer means the generic type appears as a return type in any functions or properties of the object. You can get T’s out of a List, for instance.
Consumer means the generic type appears as a parameter of any functions or as the type of any var properties of the object. You can put T’s into a MutableList, for example.
Since List produces but doesn’t consume (it doesn’t have any functions with T as a parameter), its type is marked as producing-only, aka covariant, aka out right at the declaration site so its type can always be assumed to be out wherever it’s used even if the out keyword is not used.
Since the List type is always covariant out, any List can be safely upcast to a List where the type is a supertype of the originating type. A List<String> can be cast to List<CharSequence> because any item you get out of it (anything it produces) is going to be a String, and therefore also qualifies as the supertype CharSequence.
The reverse logic would apply for something that is purely a consumer with the type marked in, but it’s harder to come up with a simple example where you would actually have a useful object like this.
A MutableList both produces and consumes, so it is invariant by default, but since it is also a List, a MutableList<String> could be safely cast to a List<CharSequence>. If you have a reference to the List<CharSequence>, you can get CharSequences out of it. The underlying object might continue to have new Strings put into it from the original reference.
Related
I found the new value class been
I found the purpose is like :
value class adds attribute to a variable and constraint it’s usage.
I was wondering what is some practical usage of value class.
Well, as stated in the documentation Kotlin Inline classes
Sometimes it is necessary for business logic to create a wrapper around some type. However, it introduces runtime overhead due to additional heap allocations. Moreover, if the wrapped type is primitive, the performance hit is terrible, because primitive types are usually heavily optimized by the runtime, while their wrappers don't get any special treatment.
To solve such issues, Kotlin introduces a special kind of class called an inline class. Inline classes are a subset of value-based classes. They don't have an identity and can only hold values.
A value class can be helpful when, for example, you want to be clear about what unit a certain value uses: does a function expect me to pass my value in meters per second or kilometers per hour? What about miles per hour? You could add documentation on what unit the function expects, but that still would be error-prone. Value classes force developers to use the correct units.
You can also use value classes to provide clear means for other devs on your project on doing operations with your data, for example converting from one unit to another.
Value classes also are not assignment-compatible, so they are treated like actual new class declarations: When a function expects a value class of an integer, you still have to pass an instance of your value class - an integer won't work. With type aliases, you could still accidentally use the underlying type, and thus introduce expensive errors.
In other words, if you simply want things to be easier to read, you can just use type aliases. If you need things to be strict and safe in some way, you probably want to use value classes instead.
I'm learning Kotlin, but I can't seem to find straight answers to simple questions. I presume that it's so new, no one has had a chance to ask the obvious questions yet. So here it goes.
When I want to get the smallest item in a range, I type:
range.start
But I get the warning, "Could be replaced with unboxed first". Not sure what unboxed means--can't even guess. But when I use this command:
range.first
the warning goes away. What's happening here? Should I even be concerned? Why does Kotlin have both a start and a first?
Boxing and unboxing refers to wrapping a primitive value in a class so it can be used with generic classes and functions or as a nullable. In Java, this is more transparent because the primitive and boxed versions of each type of variable have different names (i.e. int and Integer), whereas in Kotlin this is not very obvious. If your variable is nullable, like Int?, it is always boxed, but if it's non-nullable, it's only boxed if it's passed to a function that's generic or requests a nullable version. So boxing as a verb refers to the variable getting wrapped in a class at the moment it is passed to something that requires a boxed version.
There is an interface for a generic range called ClosedRange. When you are working with integer ranges, you are using a class called IntRange that also implements ClosedRange<Int>.
When you use the properties of the generic interface like start, the JVM has to box and unbox your Int value. This is because generics cannot be used with non-boxed primitives. There is a small amount of runtime overhead to box and unbox the primitive.
The actual class IntRange stores the values for the start and end of the range as primitives, so if you access them directly with first, you bypass the boxing that occurs if you go through the generic interface property, for a small performance gain.
In the vast majority of cases, the performance difference will be negligible anyway, but the default code inspection recommends you to use the more performant way.
Just a quote from hack documentation :
Legacy Vector, Map, and Set
These container types should be avoided in new code; use dict,
keyset, and vec instead.
Early in Hack's life, the library provided mutable and immutable
generic class types called: Vector, ImmVector, Map, ImmMap, Set, and
ImmSet. However, these have been replaced by vec, dict, and keyset,
whose use is recommended in all new code. Each generic type had a
corresponding literal form. For example, a variable of type
Vector might be initialized using Vector {22, 33, $v}, where $v
is a variable of type int.
I wonder why this change was made.
I mean, one of PHP weaknesses is that it has bad oop standard library.
Ex : str_replace and array_values methods are outside of the string/array type itself. The PHP standard library is not consistent, sometimes we must pass the array as the first parameter, other times as the second...
I was glad to see that Hack introduced true OOP encapsulation for collections.
Do you know why they stepped back and wrote utility classes such as C\, Dict\, Keyset\ and Vec\ ?
Will there be in the future an addition to add methods to built-in types (ex : Str\starts_with => "toto"->startsWith("t")) ?
Based on Dwayne Reeves' blog post introducing HSL, it seems that the main advantage is the fact that arrays are native values, not objects. This has two important consequences:
For users, the semantics are different when the values cross through arguments. Objects are passed as references, and mutations affect the original object. On the other hand, values are copied on write after passing through arguments, so without references (which are finally to be completely banned in Hack) the callee can't mutate the value of the caller, with the exception of the much stricter inout parameters.
The article cites the invariance of the mutable containers (Vector, Set, etc.) and generally how shared mutable state couples functions closer together. The soundness issues as discussed in the article are somewhat moot because there were also immutable object containers (ImmVector, ImmSet, etc.), although since these interfaces were written in userland, variance boxed the function type signature into tight constraints. There are tangible differences from this: ImmMap<Tk, +Tv> is invariant in Tk solely because of the (function(Tk): Tv) getter. Meanwhile, dict<+Tk, +Tv> is covariant in both type parameters thanks to the inherent mutation protection from copy-on-write.
For the compiler, static values can be allocated quickly and persist over the lifetime of the server. Objects on the other hand have arbitrarily complicated construction routines in general, and the collection objects weren't going to be special-cased it seems.
I will also mention that for most use cases, there is minimal difference even in code style: e.g. the -> reference chains can be directly replaced with the |> pipe operator. There is also no longer a boundary between the privileged "standard functions" and custom user functions on collection types. Finally, the collection types were final of course, so their objective nature didn't offer any actual hierarchical or polymorphic advantages to the end user anyways.
Using c++ CLI, is it recommended not to use tracking handle for value class?
for example
value class Point {
};
Point p;
or Point ^p;
C++/CLI permits that syntax, unfortunately, it cannot be expressed directly in other managed languages. You end up with the value getting boxed in an object and stored on the GC heap. Every assignment will box, reading the value unboxes it again. That's quite expensive and 99.9% of the time is the wrong thing to do. The point of value types is to make your code fast, avoiding the extra indirection through an object reference and taking advantage of processor registers. A value type value like Point fits in two registers.
By declaring it as a handle, you get the disadvantage of a ref class but add the expense of having to unbox the value every time you retrieve a member of the value type. It therefore makes no sense to do this at all, if you need a Point class with reference type semantics then just declare a ref class Point and entirely avoid the un/boxing cost. C++/CLI has a few design flaws, induced by trying make it match native C++ semantics. This is one of them.
So no, this is not recommended.
I have a class that is a leaf in the composite pattern. This class has a property that can be either of type A or type B. Their only common interface is of type Object.
How should I support this.
I can
have a add method for each type. That would however mean that I should have two properties of type A and B and should check for null when I want to get the right property.
have one property of type of Object. That would mean I had to check to see which kind of instance it is when I get the property.
What is the best solution for this type of problem? Or any better solutions?
Personally I would choose the single Object property approach. Document what types of objects the property may return, and let the calling code use the available language features to determine the object type, and cast as necessary. Implementing two properties is kinda reinventing the "is-a" operator of your language, and will quickly become unmanageable if you ever need to add more possible types.
Well if you are using a language that supports type abstraction (like Generics in Java or Templates in C++) you can just set that property as a generic type. If not, use Object, Having a method for each type is just an ugly hack (and unmaintanable, if you add more types later).