Question

Can’t figure out why this does not work:

def f[A: Double](x: A) = x / 4 // error: "Double does not take type parameters"

println(f(87.7))

While this one works:

def f[A <: Double](x: A) = x / 4

println(f(87.7))

The only difference is that in the first case I just specify particular type, and in the second I define type upper bound.

Answer 1

That’s because [A: Double] is not a correct way to define a type parameter. This form is applicable to define a class manifest context bound (new in Scala 2.8 – thanks @mhs for the link); however, the ClassManifest[T] type used to represent context bounds is a parameterized type, so Double is not fit as a class manifest.

Note though that context bounds and manifests were introduced to solve the problem of generic array creation, so there is no point using one here, as your function – as it is shown above – has nothing to do with arrays.