Scala 类型参数子句中的广义约束？

SLS指定as的语法

将扩展到

def f[T](implicit ev: T =:= 42) = ???

类似于上下文绑定

def f[T: Numeric] = ???

扩展到

def f[T](implicit ev: Numeric[T]) = ???

在2.13中（如果您对约束单例感到好奇，它支持单例类型），您可以执行以下操作：

@ import $plugin.$ivy.`org.typelevel:kind-projector_2.13.1:0.11.0`
import $plugin.

@ type a = 23
defined type a

@ def f[N : * =:= a]: Unit = ()
defined function f

@ f[a]


@ f[23]


@ f[25]
cmd9.sc:1: Cannot prove that 25 =:= Int(23).
val res9 = f[25]
            ^
Compilation Failed

@ def g[N : * =:= 16]: Unit = ()
defined function g

@ g[16]


@ g[23]
cmd11.sc:1: Cannot prove that 23 =:= 16.
val res11 = g[23]
             ^
Compilation Failed

所以，是的，这似乎是可能的。你只需要使用投影仪来应用第二个参数

使用

def f[T](implicit ev: Numeric[T]) = ???

@ import $plugin.$ivy.`org.typelevel:kind-projector_2.13.1:0.11.0`
import $plugin.

@ type a = 23
defined type a

@ def f[N : * =:= a]: Unit = ()
defined function f

@ f[a]


@ f[23]


@ f[25]
cmd9.sc:1: Cannot prove that 25 =:= Int(23).
val res9 = f[25]
            ^
Compilation Failed

@ def g[N : * =:= 16]: Unit = ()
defined function g

@ g[16]


@ g[23]
cmd11.sc:1: Cannot prove that 23 =:= 16.
val res11 = g[23]
             ^
Compilation Failed

@ def h[N : * <:< 16]: Unit = ()
defined function h

@ h[16]


@ h[17]
cmd13.sc:1: Cannot prove that 17 <:< 16.
val res13 = h[17]
             ^
Compilation Failed