understanding scala : currying

Question

I recently started learning Scala and came across currying. From an answer in this post, this code snippet

def sum(a: Int)(b: Int) = a + b

expands out to this

def sum(a: Int): Int => Int = b => a + b

Then I saw a snippet from scala-lang, which shows it's possible to write something like this to emulate a while loop

 def whileLoop (cond : => Boolean) (body : => Unit) : Unit = { if (cond) { body whileLoop (cond) (body) } }

Out of curiosity, I tried to expand this out, and got this

 def whileLoop2 (cond : => Boolean) : (Unit => Unit) = (body : => Unit) => if (cond) { body whileLoop2 (cond) (body) }

But there seems to be some syntax that I'm missing because I get an error

error: identifier expected but '=>' found. (body : => Unit) => ^

What is the proper way to expand out the emulated while loop?

0__ · Accepted Answer · 2014-09-29 21:51:08Z

The tricky part is dealing with the parameterless function or "thunk" type => Unit. Here is my version:

def whileLoop2 (cond: => Boolean): (=> Unit) => Unit = body => if (cond) { body whileLoop2 (cond)(body) } var i = 5 val c = whileLoop2(i > 0) c { println(s"loop $i"); i -= 1 }

It appears that you can annotate the return type with (=> Unit) => Unit, but you cannot annotate (body: => Unit), so you have to rely on the type inference here.

Is it possible change this solution such that i can write whileLoop2 (i > 0) { println "..." } instead of whileLoop2(i > 0) { () => println "..." } ? The example in the scala-lang page doesn't have the extra () => bit when using their emulated while loop.
Sorry, it is possible to use parameterless function arguments. See my edited version.

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