What is a "context bound" in Scala?

context-bound, scala, scala-2.8

Solution

Did you find this article? It covers the new context bound feature, within the context of array improvements.

Generally, a type parameter with a context bound is of the form `[T: Bound]`; it is expanded to plain type parameter `T` together with an implicit parameter of type `Bound[T]`.

Consider the method `tabulate` which forms an array from the results of applying a given function f on a range of numbers from 0 until a given length. Up to Scala 2.7, tabulate could be written as follows:

def tabulate[T](len: Int, f: Int => T) = {
    val xs = new Array[T](len)
    for (i <- 0 until len) xs(i) = f(i)
    xs
}

In Scala 2.8 this is no longer possible, because runtime information is necessary to create the right representation of `Array[T]`. One needs to provide this information by passing a `ClassManifest[T]` into the method as an implicit parameter:

def tabulate[T](len: Int, f: Int => T)(implicit m: ClassManifest[T]) = {
    val xs = new Array[T](len)
    for (i <- 0 until len) xs(i) = f(i)
    xs
}

As a shorthand form, a context bound can be used on the type parameter `T` instead, giving:

def tabulate[T: ClassManifest](len: Int, f: Int => T) = {
    val xs = new Array[T](len)
    for (i <- 0 until len) xs(i) = f(i)
    xs
}

Problem

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