scala - When do you have to include the abstract type parameter? -


here's 2 piece of example code, found here: 

scala> val im = m.reflect(new c) im: reflect.runtime.universe.instancemirror = instance mirror c@3442299e

and here:

scala> def mkarray[t : classtag](elems: t*) = array[t](elems: _*)

the first piece of code uses method defined in scala.reflect.api.mirrors (found here):

abstract def reflect[t](obj: t)(implicit arg0: classtag[t]): universe.instancemirror

if notice, there's classtag used classtag[t] , 1 used classtag. what's reason difference?

for part, both equivalent.

foo[t: classtag]()

is syntactic sugar for

 foo[t]()(implicit ct: classtag[t]

however, difference between these signatures in former, have access classtag via implicitly[classtag[t]] while in latter, can use ct

syntax change note: before 2.10.x used not possible have have both context bound classtag , implicit argument list, such as:

foo[t: classtag]()(implicit ex: executioncontext)

the error used reported "error: cannot have both implicit parameters , context bounds". intellij 13 still reporting late 2.10.4, accepted scala compiler.


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