Scala functional composition - scala

Question

I'm trying to get a function currying working correctly. What I have is the following:
def method(x: ByteArrayInputStream)
(y: ByteArrayOutputStream)
(z: GZIPOutputStream)
(func: (ByteArrayInputStream, GZIPOutputStream) => Unit) = {
.....
.....
}
Now when I call it, I call it like this:
method(new ByteArrayInputStream("".getBytes("UTF-8")))
(new ByteArrayOutputStream())
(new GZIPOutputStream(_))
(myFunc(_, _))
My understanding is that in the third parameter i.e., to the GZIPOutputStream, when I say _, it will pick the value from the second parameter. But it complains saying that
Type mismatch, expected: GZIPOutputstream, actual: (OutputStream) => GZIPOutputStream
Any hints?

Answer 1

The problem is at
(new GZIPOutputStream(_))
As your error says, your method wants a a GZIPOutputstream, but you are passing it a function from OutputStream to GZIPOutputStream
The underscore is a little confusing at first, but it is the way to tell scala that you are intentionally not passing an argument to GZIPOutputStream so that it won't complain about missing arguments. In other words, you are passing the function itself instead of the result of the function.
How to fix it depends on what you're actually trying to do. If you actually want to pass a GZIPOutputStream, you'll need to replace that _ with an OutputStream.
If your intent is to have method create a GZIPOutputStream given a factory function like the one you are passing, you'd want to change the declared type for z. Eg,
(z: (OutputStream) => GZIPOutputStream)
and then in the method body you could say something like z(y) to get a GZIPOutputStream. (Or replace y with some other output stream.)

Answer 2

I'm not exactly sure how to do this... but here is one solution that mimics what you are looking for
def add(j: Int)(i: Option[Int] = None): Int = j + i.getOrElse(j)
add(5)()
The add(5)() returns 10 and uses the j value

Answer 3

I have managed to skin that a bit and here is what I have been to:
val bytePayload = method(new ByteArrayInputStream(s.getBytes("UTF-8")))(new ByteArrayOutputStream())(writeBytes(_,_))
def method(bin: ByteArrayInputStream)
(bos: ByteArrayOutputStream)
(func: (ByteArrayInputStream, GZIPOutputStream) => Unit): Either[String, Array[Byte]] = {
val gzip = new GZIPOutputStream(bos)
try {
func(bin, gzip)
gzip.finish
} catch {
case e: Exception => Left(e.getMessage)
} finally {
bin.close()
bos.close()
gzip.close()
}
Right(bos.toByteArray)
}
Though I still handle Exceptions, I'm to some extent convinced that I don't throw them around.