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Many shapes and the same function

12 Jan 2017

I was playing around with a small function in Scala and found it kind of interesting that you can express the same thing in different ways. We are writting a function that will fold over a string an accumulate on running parensToFloor a function that looks like:

def parensToFloor(c:Char): Int = ???

The most common way to write this function would be:

def myFn1(s:String) : Int = {
	s.foldLeft(0)((x,y) => x + parensToFloor(y))
}

What would it look like point free?

I arrived to a few options that didn’t work :

def myFnx(s:String) : Int = {
    s foldLeft 0 { _ + parensToFloor(_)}
//>error: Int(0) does not take parameters
//              s foldLeft 0 { _ + parensToFloor(_)}
//                          ^
}

this made sense to me but the following one…mmm I imagine the compiler sees that since foldLeft is curried then it makes sense but what does the compiler want?

def myFnx(s:String) : Int = {
    s foldLeft 0 apply { (x,y) =>_ + parensToFloor(_)}
// >error: missing arguments for method foldLeft in trait IndexedSeqOptimized;
//follow this method with `_' if you want to treat it as a partially applied function
//             s foldLeft 0 apply { (x,y) =>_ + parensToFloor(_)}
//              ^
}

so thinking about that and how to curry, I started trying to add parens around and see what happens and this worked:

    (s foldLeft 0) { _ + parensToFloor(_)}

Something that worked but it wasn’t what I was looking for (it needed a change to the definition of parensToFloor):


def parensToFloor(accumulated:Int, c:Char):Int = ???

//... more stuff.

def myFnx(s:String) : Int = {
// first the non working version same error as before
s foldLeft 0 parensToFloor

// than later evolved to because apply was missing
(s foldLeft 0) { parensToFloor }

then since we are here, might as well go and try and see what it looks like with operators (from a coworker):

(0 /: s) { _ + parensToFloor(_) }

An interesting thing is that the usage of parens and braces is something I still find confusing in time in Scala. The source of my confusion is not totally unfounded, in the docs we can see that:

however here we see that blocks can also be expressions…

I’ll have to leave this here. More to follow…

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