[英]How to help Scalaz with type inference and 2 type parameters
I have something called a Generator
: 我有一个叫做Generator
东西:
trait Generator[A, B] {
def generate(in: Seq[A]): Seq[B]
}
I can provide a Bind
instance for this generator: 我可以为此生成器提供一个Bind
实例:
object Generator {
implicit def generatorBind[T]: Bind[({type l[B] = Generator[T, B]})#l] = new Bind[({type l[B] = Generator[T, B]})#l] {
def map[A, B](generator: Generator[T, A])(f: A => B): Generator[T, B] = new Generator[T, B] {
def generate(in: Seq[T]): Seq[B] = generator.generate(in).map(f)
}
def bind[A, B](generator: Generator[T, A])(f: A =>Generator[T, B]): Generator[T, B] = new Generator[T, B] {
def generate(in: Seq[T]): Seq[B] = generator.generate(in).flatMap(v => f(v).generate(in))
}
}
}
Unfortunately, type inference is completely lost if I try to use my generators as applicative instances: 不幸的是,如果我尝试将生成器用作应用实例,则类型推断将完全丢失:
val g1 = new Generator[Int, Int] { def generate(seq: Seq[Int]) = seq.map(_ + 1) }
val g2 = new Generator[Int, Int] { def generate(seq: Seq[Int]) = seq.map(_ + 10) }
// doesn't compile
// can make it compile with ugly type annotations
val g3 = ^(g1, g2)(_ / _)
My only workaround for now has been to add a specialised method to the Generator
object: 我目前唯一的解决方法是向Generator
对象添加专门的方法:
def ^[T, A, B, C](g1: Generator[T, A], g2: Generator[T, B])(f: (A, B) => C) =
generatorBind[T].apply2(g1, g2)(f)
Then this compiles: 然后编译:
val g4 = Generator.^(g1, g2)(_ / _)
Is there a workaround for this problem? 有解决此问题的方法吗? I suppose there is because using State[S, A]
as a Monad
poses the same kind of issue (but in Scalaz there seems to be a special treatment for State
). 我想是因为将State[S, A]
用作Monad
引起同样的问题(但在Scalaz中似乎对State
有特殊的处理)。
You can use ApplicativeBuilder if explicitly annotate g1
and g2
types, or change to abstract class Generator
如果显式注释g1
和g2
类型,则可以使用ApplicativeBuilder ,也可以更改为abstract class Generator
// java.lang.Object with Generator[Int, Int] !!!
val badInference = new Generator[Int, Int] { def generate(seq: Seq[Int]) = seq.map(_ + 1) }
val g1: Generator[Int, Int] = new Generator[Int, Int] { def generate(seq: Seq[Int]) = seq.map(_ + 1) }
val g2: Generator[Int, Int] = new Generator[Int, Int] { def generate(seq: Seq[Int]) = seq.map(_ + 10) }
val g3 = (g1 |@| g2)(_ / _)
I think implicit macro's fundep materialization (aka functional dependency) is to help this. 我认为隐式宏的Fundep实现 (也称为功能依赖)可以帮助实现这一点。
trait Iso[T, U] {
def to(t: T) : U
def from(u: U) : T
}
case class Foo(i: Int, s: String, b: Boolean)
def conv[C](c: C)(implicit iso: Iso[C, L]): L = iso.from(c)
val tp = conv(Foo(23, "foo", true))
It requires macro paradise tho. 它需要宏的天堂。
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