Fred Haslam Fred Haslam - 1 month ago 9
Scala Question

When is a scala partial function not a partial function?

While creating a map of String to partial functions I ran into unexpected behavior. When I create a partial function as a map element it works fine. When I allocate to a val it invokes instead. Trying to invoke the check generates an error. Is this expected? Am I doing something dumb? Comment out the

check()
to see the invocation. I am using scala 2.7.7

def PartialFunctionProblem() = {
def dream()() = {
println("~Dream~");
new Exception().printStackTrace()
}
val map = scala.collection.mutable.HashMap[String,()=>Unit]()
map("dream") = dream() // partial function
map("dream")() // invokes as expected
val check = dream() // unexpected invocation
check() // error: check of type Unit does not take parameters
}

Answer

For convenience, Scala lets you omit empty parens when calling a method, but it's clever enough to see that the expected type in the first case is ()=>Unit, so it doesn't remove all the parens for you; instead, it converts the method into a function for you.

In the val check case, however, it looks just like a function call result getting assigned to a variable. In fact, all three of these do the exact same thing:

val check = dream
val check = dream()
val check = dream()()

If you want to turn the method into a function, you place _ after the method in place of the argument list(s). Thus,

val check = dream() _

will do what you want.