CODEWITHSUNDEEP
scala
Ra Ka
Ra Ka

Reputation: 3055

Understanding multiple parameter types in Scala Type Parameterization

I understand the following is an example of upper bound type parameterization in scala, where T must be a subtype of Command. 

def getSomeValue[T <: Command] = ...

However, Today I found the following implementation of type parameterization with multiple parameter types and as I'm a beginner in scala, I'm quite unable to understand what actually it does. Is this means T must be subtype of either Command, Foo or Bar?

def getSomeValue[T <: Command : Foo : Bar] = ...

Upvotes: 3

Views: 215

Answers (2)

jwvh
jwvh

Reputation: 51271

The stand-alone colon, in a type parameter specification, is actually shorthand (syntactic sugar) for a longer, and rather complicated, parameter invocation.

Example: def f[N: Numeric](n: N) = ...

is really: def f[N](n: N)(implicit ev: Numeric[N]) = ...

Meaning that when f(x) is invoked there has to be an implicit in scope that matches Numeric[x.type].

So, in your example code fragment: def getSomeValue[T <: Command : Foo : Bar] = ...

The compiler turns that into, roughly, the following:

def getSomeValue[T <: Command](implicit ev1: Foo[T], ev2: Bar[T]) = ...

We can demonstrate this by providing just enough skeletal code to actually make it compilable.

class Command {}
class Foo[F] {}
class Bar[B] {}
class Cmd extends Command

def getSomeValue[T <: Command : Foo : Bar](t: T) = t

getSomeValue(new Cmd)(new Foo[Cmd], new Bar[Cmd])
                  // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                  // implicit parameters supplied explicitly

Upvotes: 3

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149618

Is this means T must be subtype of either Command, Foo or Bar?

No, the upper bound is still required for T. You now must also have an implicit instance of Foo[T] and Bar[T] in scope when this method is called. This is called Context Bounds, and it syntax sugar for:

def getSomeValue[T <: Command](implicit f: Foo[T], b: Bar[T])

If you're unaware of what implicit means, refer to the documentation for more.

Upvotes: 3

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