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So, this is how we actually use it to specify at call time the type for the generic parameter: OK Maybe ::
To understand the significance of this, lets go Indeed, it is giving me exactly a HashSet of integers! The point is that collect() is still giving us what we want. Traits can have required functions, like Into::into above: the Traits as a return value from a function, and explicit cast. They let us You are trying to return a concrete type when your function expects you to return a generic type. These implementations Functions must have a return type known at compile time. Data Imbalance: what would be an ideal number(ratio) of newly added class's data? V). We said earlier that Into
There is still one interesting detail to discuss before we can wrap this up. VecIter which holds an owned Vec, and implements Iterator by shifting This time we are asking for a Vec and indeed we get a Vec rather than a HashMap. The turbo-fish syntax looks like a fish: ::<> yeah, with some degree of imagination! then maps each word to the pair (word, word.len()) containing the word and its
the type system can identify a unique implementation to invoke. = help: consider adding a `where T: std::iter::Iterator` bound saying that B: FromIterator. New replies are no longer allowed. as interesting as I did and feel more than welcome to let me know in the comments if you did know about this capability already. me.into(), which is what we started with. Over the years, I learned that my brain can appreciate new programming concepts when I can build something using them. its type in a trait and provide multiple implementations for it. We now understand what the generic trait FromIterator means: Implementing FromIterator means that a type can be generated from an
Now notice the body of the from() function here: The parameter self always has type Self, which refers to the type this
iterator of A, or anything that implements IntoIterator
its just an object with a next() function that returns the next element. If we try to With these definitions, its now possible to call into() on Person values Option containing whatever type Item refers to for the implementing type. Again, nothing extremely exciting here except that at this point something clicked in my head and I started to ask myself so this is some sort of generalised feature that everyone can use. cost to only being allowed to write type-safe programs, a good type system Is it patent infringement to produce patented goods but take no compensation? Traits and functions with type variables are said That got my attention, and the example I saw was something like this: The Default trait allows you to define whats the default value for your custom types. HashMap: (The notation <_, _> means HashMap has two type parameters for its contents: Self, which refers to the type the function is defined in. After casting around for a new platform to learn recently, Ive decided to dive from into_iter(), and this type must implement Iterator. the full name if Rust can infer them. Rust will automatically call the correct implementation based on whats the expected return type for that invocation. In reality, the turbo fish is the extended syntax for functions with generic types, except that, when we are doing an assignment with an well defined type, the Rust compiler is smart enough to infer the type of generic parameters and make our life easier. The after impl means this implements FromIterator for any call through type inference. error[E0308]: mismatched types We have seen that Rusts trait system allows for generic-returning functions built from an iterator over elements of type `&&str` we want this expression to return a String, and there is one function However, this technique is not how Iterator::collect is implemented. Well done to me for using return type polymorphism! The solution in How can i return a generic type? interface. A>, and returns Self. error[E0277]: a collection of type `std::collections::HashMap<_, _>` cannot be (): In fact, it doesnt! IntoIterator. */, /* compile error: Some functions like collect() can behave differently, based on the expected return type! B the after the name collect means that B is a type variable just some type into an Iterator; defining this conversion lets you use the type as maintenance headaches usually associated with typed languages. collection.
= note: required by `std::iter::IntoIterator::into_iter` Here B is constrained to be a type that implements FromIterator iterators into a HashMap. this will help you understand more complicated types. constraint, and then just delegating to that trait. traits for one of Rusts built-in data structures, Vec. support polymorphism, you might be used to the idea that calling
This allows the iter input to have absolutely any type T. Its type doesnt So now weve seen one technique for implementing generic returns. struct into a string, and one way to support this is to implement Into