type-bounds

Scala: overriding type member with bounds

由 爱⌒轻易说出口 提交于 2019-12-24 03:25:51
问题 I've narrowed down my issue to the following minimal (non-)working example: class Z trait A[E <: Z] { type T[X <: E] <: A[X] } trait B[E <: Z] extends A[E] { type T[X <: E] <: B[X] } trait C[E <: Z, F[X <: Z] <: C[X, F]] extends A[E] { type T[X <: E] = F[X] } class D[E <: Z] extends B[E] with C[E, D] It is sort of a higher-kinded F-bounded polymorphism (I'm using T[E] as the return value of some methods). When I try to compile this code I get: error: overriding type T in trait B with bounds[X

How to set lower-bound to type parameters in TypeScript?

由 偶尔善良 提交于 2019-12-23 19:08:07
问题 In TypeScript (my version: 2.1.1), it is okay to set an upper-bound to type parameters like this: class Animal {} class Cat extends Animal {} class Kitten extends Cat{} function foo<A extends Animal>(a: A) { /* */ } How can I set a lower-bound on A ? The following code does not work: class Animal {} class Cat extends Animal {} class Kitten extends Cat{} function foo<A super Kitten>(a: A) { /* */ } It will cause a compilation error. People in the GitHub issue #13337 of the TypeScript repo have

How does the type inferencer work on reduceLeft?

由 我们两清 提交于 2019-12-22 10:14:31
问题 Further to my other question about reduceLeft , the signature of reduceLeft on Seq is def reduceLeft [B >: A] (f: (B, A) ⇒ B): B and we can call it with expressions such as List(1,2,3,4) reduceLeft (_ + _) In this example A is Int , so reduceLeft expects a Function2[B >: Int, Int, B] . Regardless of how reduceLeft works (which is irrelevant), how does the type inferencer know that B has a + method, when it could be of type Any ? 回答1: I think the section 6.26.4 Local Type Inference of the spec

Why doesn't type inference work here?

由 独自空忆成欢 提交于 2019-12-22 05:22:01
问题 This problem arose in a module I'm writing, but I have made a minimal case that exhibits the same behaviour. class Minimal[T](x : T) { def doSomething = x } object Sugar { type S[T] = { def doSomething : T } def apply[T, X <: S[T]] (x: X) = x.doSomething } object Error { val a = new Minimal(4) Sugar(a) // error: inferred [Nothing, Minimal[Int]] does not fit the bounds of apply Sugar[Int, Minimal[Int]](a) // works as expected } The problem is that the compiler manages to figure out the inner

Why doesn't type inference work here?

由 ╄→гoц情女王★ 提交于 2019-12-22 05:21:39
问题 This problem arose in a module I'm writing, but I have made a minimal case that exhibits the same behaviour. class Minimal[T](x : T) { def doSomething = x } object Sugar { type S[T] = { def doSomething : T } def apply[T, X <: S[T]] (x: X) = x.doSomething } object Error { val a = new Minimal(4) Sugar(a) // error: inferred [Nothing, Minimal[Int]] does not fit the bounds of apply Sugar[Int, Minimal[Int]](a) // works as expected } The problem is that the compiler manages to figure out the inner

Is there special meaning to an underscore (_) in Type Bounds?

由 左心房为你撑大大i 提交于 2019-12-21 02:22:11
问题 I'm trying to understand Scala's existential types. Is there any difference between: def foo[X <: Bar] = 3 and def foo[_ <: Bar] = 3 or are they something more than just unnamed type parameters? 回答1: Here _ is indeed just an unnamed type parameter, no more, no less. There is no difference between def foo[_ <: Bar] = 3 and def foo[X <: Bar] = 3 where X is unused. UPDATE : In response to: "I can't think of a use case for an unused type, I'd be grateful for one": Note that this is pretty much

Cannot override a type with non-volatile upper bound

由 ﹥＞﹥吖頭↗ 提交于 2019-12-20 20:12:06
问题 I have a compiler error in scala and I don't know what does it refer to: Assume these declarations: trait Abstract { type MyType } trait AInner trait A extends Abstract{ type MyType <: AInner } trait BInner { def bMethod : Int } trait B extends Abstract with A{ override type MyType <: BInner with A#MyType } What I'm trying to achieve here(in trait B ) is to further restrict the type MyType declared in Abstract , so any value of type MyType must extend all the MyType s in the mixin tree. The

Types in Scala - lower bounds

由 回眸只為那壹抹淺笑 提交于 2019-12-12 01:25:58
问题 on code below. My expectation is that T must be a of type B or A , so call to lowerBound(new D) should probably not compile (?). Similar experiments with upperbound give me expected typecheck errors. Thanks for giving the hint. object varianceCheck { class A { override def toString = this.getClass.getCanonicalName } class B extends A class C extends B class D extends C def lowerBound[T >: B](param: T) = { param } println(lowerBound(new D)) //> varianceCheck.D } 回答1: With your implementation

Upper and Lower bound on scala type

由 不羁的心 提交于 2019-12-10 15:50:35
问题 Consider the following hierarchy: class C1 class C2 extends C1 class C3 extends C2 class C4 extends C3 I want to write a function that just accepts types C2 and C3 . For that I thought of the following: def f [C >: C3 <: C2](c :C) = 0 I'd expect the following behaviour f(new C1) //doesn't compile, ok f(new C2) //compiles, ok f(new C3) //compiles, ok f(new C4) // !!! Compiles, and it shouldn't The problem is when calling it with C4 , which I don't want to allow, but the compiler accepts. I

How does the type inferencer work on reduceLeft?

由 Deadly 提交于 2019-12-05 19:22:50
Further to my other question about reduceLeft , the signature of reduceLeft on Seq is def reduceLeft [B >: A] (f: (B, A) ⇒ B): B and we can call it with expressions such as List(1,2,3,4) reduceLeft (_ + _) In this example A is Int , so reduceLeft expects a Function2[B >: Int, Int, B] . Regardless of how reduceLeft works (which is irrelevant), how does the type inferencer know that B has a + method, when it could be of type Any ? I think the section 6.26.4 Local Type Inference of the spec sort of explains what's going on. The compiler will look for an optimal type. When the type parameter is
订阅 type-bounds