Scala中的一些特殊字符用法

<:< and <%< work similarly, but with slight variations:

• A =:= B means A must be exactly B
• A <:< B means A must be a subtype of B (analogous to the simple type constraint <:)
• A <%< B means A must be viewable as B, possibly via implicit conversion (analogous to the simple type constraint <%)