[SLS将type parameter clause的语法指定为
TypeParamClause ::= ‘[’ VariantTypeParam {‘,’ VariantTypeParam} ‘]’
FunTypeParamClause::= ‘[’ TypeParam {‘,’ TypeParam} ‘]’
VariantTypeParam ::= {Annotation} [‘+’ | ‘-’] TypeParam
TypeParam ::= (id | ‘_’) [TypeParamClause] [‘>:’ Type] [‘<:’ Type] {‘<%’ Type} {‘:’ Type} {‘<%’ Type} {‘<%’ Type}
我们看到>:
,<:
,<%
,<%
,:
作为类型形参子句中允许的保留名称。有没有一种方法可以在类型参数子句中使用generalised type constraint符号名<:<
,=:=
,例如
def f[T =:= 42] = ???
将扩展为
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(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,似乎有可能。您只需要使用同类的投影仪即可应用第二个参数。
<:<
应该是同一故事:
@ 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