generics – w3toppers.com

How to use TypeToken + generics with Gson in Kotlin

Create this inline fun: inline fun <reified T> Gson.fromJson(json: String) = fromJson<T>(json, object: TypeToken<T>() {}.type) and then you can call it in this way: val turns = Gson().fromJson<Turns>(pref.turns) // or val turns: Turns = Gson().fromJson(pref.turns) Previous Alternatives: ALTERNATIVE 1: val turnsType = object : TypeToken<List<Turns>>() {}.type val turns = Gson().fromJson<List<Turns>>(pref.turns, turnsType) You have to put … Read more

Generics Error: expected type parameter, found struct

First error: you create a Rustmark object with the field parser of type DefaultParser and the field renderer of type HTMLRenderer, but the function is expected to return Rustmark <P, R>. In general P is not of type DefaultParser and R is not of type HTMLRenderer, so it will never compile. A good solution if … Read more

Why can’t `&(?Sized + Trait)` be cast to `&dyn Trait`?

This problem can be reduced to the following simple example (thanks to turbulencetoo): trait Foo {} fn make_dyn<T: Foo + ?Sized>(arg: &T) -> &dyn Foo { arg } At first glance, it really looks like this should compile, as you observed: If T is Sized, the compiler knows statically what vtable it should use to … Read more

Generic higher order function

Here’s the inline magic. Let’s take kvb’s code and define a single gmap function that handles all cases: let inline gmap f (x, y) = f $ x, f $ y type One = One with static member ($) (One, x) = 1 // Example1 ConvertAll type Id = Id with static member ($) (Id … Read more

Scala 2.8 CanBuildFrom

Note that the second argument to map is an implicit argument. There must be an implicit in scope with the appropriate types, or, otherwise, you must pass such an argument. In your example, That must be Set[String], B must be Int and Repr must be List[String]. Therefore, for that to compile you need the following … Read more

Generic repository in ASP.NET Core without having a separate AddScoped line per table in Startup.cs?

Just use the non-generic registration overloads (the ones where you need to pass the 2 Type objects.) Then provide the open generic types of both your interface and the implementation: services.AddScoped(typeof(IGenericRepository<>), typeof(GenericRepository<>)); In your controller, add a dependency for a repository of a specific type (a closed generic type): public HomeController(IGenericRepository<Lookup1> repository) { … }

Why can’t I use a key function that returns a reference when sorting a vector with sort_by_key?

For now, you have to use the “long” form: v.sort_by(|x, y| key(x).cmp(&key(y))); Why am I getting these errors? Is there any way to fix them? The cause and fix are one-and-the same: Rust is simply not currently expressive enough to represent what you want. The feature needed is called generic associated types (GATs); previously known … Read more

Generic Functions in Go

As of Go 1.18 you can write a generic function Print as below: package main import ( “fmt” ) // T can be any type func Print[T any](s []T) { for _, v := range s { fmt.Print(v) } } func main() { // Passing list of string works Print([]string{“Hello, “, “world\n”}) // You can … Read more

Matching a generic parameter to an associated type in an impl

I must explain to the compiler that B is the same as A::Foo There is a special syntax for it: impl<A, B> Baz<A, B> where A: Generator<Foo = B>, { fn add_foo(&mut self) { self.vec.push(self.generator.generate()); } }

How do I return a Filter iterator from a function?

Rust 1.26 fn filter_one(input: &[u8]) -> impl Iterator<Item = &u8> { input.iter().filter(|&&x| x == 1) } fn main() { let nums = vec![1, 2, 3, 1, 2, 3]; let other: Vec<_> = filter_one(&nums).collect(); println!(“{:?}”, other); } Rust 1.0 fn filter_one<‘a>(input: &’a [u8]) -> Box<Iterator<Item = &’a u8> + ‘a> { Box::new(input.iter().filter(|&&x| x == 1)) } … Read more