什么是 Scala 上下文和视图边界？

Question

In a simple way, what are context and view bounds and what is the difference between them?

Some easy-to-follow examples would be great too!

Answer 1

I thought this was asked already, but, if so, the question isn't apparent in the "related" bar. So, here it is:

What is a View Bound?

A view bound was a mechanism introduced in Scala to enable the use of some type A as if it were some type B . The typical syntax is this:

def f[A <% B](a: A) = a.bMethod

In other words, A should have an implicit conversion to B available, so that one can call B methods on an object of type A . The most common usage of view bounds in the standard library (before Scala 2.8.0, anyway), is with Ordered , like this:

def f[A <% Ordered[A]](a: A, b: A) = if (a < b) a else b

Because one can convert A into an Ordered[A] , and because Ordered[A] defines the method <(other: A): Boolean , I can use the expression a < b .

Please be aware that view bounds are deprecated , you should avoid them.

What is a Context Bound?

Context bounds were introduced in Scala 2.8.0, and are typically used with the so-called type class pattern , a pattern of code that emulates the functionality provided by Haskell type classes, though in a more verbose manner.

While a view bound can be used with simple types (for example, A <% String ), a context bound requires a parameterized type , such as Ordered[A] above, but unlike String .

A context bound describes an implicit value , instead of view bound's implicit conversion . It is used to declare that for some type A , there is an implicit value of type B[A] available. The syntax goes like this:

def f[A : B](a: A) = g(a) // where g requires an implicit value of type B[A]

This is more confusing than the view bound because it is not immediately clear how to use it. The common example of usage in Scala is this:

def f[A : ClassManifest](n: Int) = new Array[A](n)

An Array initialization on a parameterized type requires a ClassManifest to be available, for arcane reasons related to type erasure and the non-erasure nature of arrays.

Another very common example in the library is a bit more complex:

def f[A : Ordering](a: A, b: A) = implicitly[Ordering[A]].compare(a, b)

Here, implicitly is used to retrive the implicit value we want, one of type Ordering[A] , which class defines the method compare(a: A, b: A): Int .

We'll see another way of doing this below.

How are View Bounds and Context Bounds implemented?

It shouldn't be surprising that both view bounds and context bounds are implemented with implicit parameters, given their definition. Actually, the syntax I showed are syntactic sugars for what really happens. See below how they de-sugar:

def f[A <% B](a: A) = a.bMethod
def f[A](a: A)(implicit ev: A => B) = a.bMethod

def g[A : B](a: A) = h(a)
def g[A](a: A)(implicit ev: B[A]) = h(a)

So, naturally, one can write them in their full syntax, which is specially useful for context bounds:

def f[A](a: A, b: A)(implicit ord: Ordering[A]) = ord.compare(a, b)

What are View Bounds used for?

View bounds are used mostly to take advantage of the pimp my library pattern, through which one "adds" methods to an existing class, in situations where you want to return the original type somehow. If you do not need to return that type in any way, then you do not need a view bound.

The classic example of view bound usage is handling Ordered . Note that Int is not Ordered , for example, though there is an implicit conversion. The example previously given needs a view bound because it returns the non-converted type:

def f[A <% Ordered[A]](a: A, b: A): A = if (a < b) a else b

This example won't work without view bounds. However, if I were to return another type, then I don't need a view bound anymore:

def f[A](a: Ordered[A], b: A): Boolean = a < b

The conversion here (if needed) happens before I pass the parameter to f , so f doesn't need to know about it.

Besides Ordered , the most common usage from the library is handling String and Array , which are Java classes, like they were Scala collections. For example:

def f[CC <% Traversable[_]](a: CC, b: CC): CC = if (a.size < b.size) a else b

If one tried to do this without view bounds, the return type of a String would be a WrappedString (Scala 2.8), and similarly for Array .

The same thing happens even if the type is only used as a type parameter of the return type:

def f[A <% Ordered[A]](xs: A*): Seq[A] = xs.toSeq.sorted

What are Context Bounds used for?

Context bounds are mainly used in what has become known as typeclass pattern , as a reference to Haskell's type classes. Basically, this pattern implements an alternative to inheritance by making functionality available through a sort of implicit adapter pattern.

The classic example is Scala 2.8's Ordering , which replaced Ordered throughout Scala's library. The usage is:

def f[A : Ordering](a: A, b: A) = if (implicitly[Ordering[A]].lt(a, b)) a else b

Though you'll usually see that written like this:

def f[A](a: A, b: A)(implicit ord: Ordering[A]) = {
    import ord.mkOrderingOps
    if (a < b) a else b
}

Which take advantage of some implicit conversions inside Ordering that enable the traditional operator style. Another example in Scala 2.8 is the Numeric :

def f[A : Numeric](a: A, b: A) = implicitly[Numeric[A]].plus(a, b)

A more complex example is the new collection usage of CanBuildFrom , but there's already a very long answer about that, so I'll avoid it here. And, as mentioned before, there's the ClassManifest usage, which is required to initialize new arrays without concrete types.

The context bound with the typeclass pattern is much more likely to be used by your own classes, as they enable separation of concerns, whereas view bounds can be avoided in your own code by good design (it is used mostly to get around someone else's design).

Though it has been possible for a long time, the use of context bounds has really taken off in 2010, and is now found to some degree in most of Scala's most important libraries and frameworks. The most extreme example of its usage, though, is the Scalaz library, which brings a lot of the power of Haskell to Scala. I recommend reading up on typeclass patterns to get more acquainted with all the ways in which it can be used.

EDIT

Related questions of interest:

• A discussion on types, origin and precedence of implicits
• Chaining implicits