Context Bounds

A context bound is a syntactic sugar for expressing a context parameter that depends of a type parameter. For example, imagine that we have the following method:

def maximum[T](xs: List[T])(using Ordering[T]): T =
  xs.reduceLeft(max)

Note: in the previous example, max is a function that has the following signature: def max[T](x: T, y: T)(using ord: Ordering[T]).

This can be rewritten using Context bounds:

def maximum[T: Ordering](xs: List[T]): T =
  xs.reduceLeft(max)

: Ordering is the bound and it indicates that we have a context parameter of type Ordering[T]. So, it is equivalent to state that we have (using Ordering[T]) as parameter.

Conclusion

In this short post we’ve seen Context Bounds which is an interesting way of expressing context parameters that relies on type parameters. This syntax is very useful when writing type classes or for expressing effect systems capabilities.