lundi 20 avril 2015

curriable function that returns a function in scala via '=>', and (secondly), via 1 arg list followed by another

Scala Experts:

I'm starting to learn a little scala, and I basically understand functions that return functions and currying, but I've seen two syntaxes for doing this, and I'd like to better understand the differences, and maybe a little of theory behind what's going on.

In the first method (using =>) I can curry the function by just specifying the argument to be bound to variable 'x'. However when I try to do this with the second approach, the compiler tells me I need to specify the '_' wild card for the second argument.

I understand what I need to do... But I am not sure why I need to do things this way. Can someone please tell me what the scala compiler is doing here ? Thanks !

First Method using =>

def add(x:Int) = (y:Int) => x + (-y)
add: (x: Int)Int => Int

scala> def adder = add(100)   // x is bound to 100 in the returned closure
adder: Int => Int

scala> adder(1)
res42: Int = 99

Second Method using one arg list followed by another

scala> def add2(x:Int)(y:Int) :  Int =  x + y
add2: (x: Int)(y: Int)Int

scala> def  adder2 = add2(100)
<console>:9: error: missing arguments for method add2;
follow this method with `_' if you want to treat it 
 as a partially applied function

       def  adder2 = add2(100)

scala> def  adder2 = add2(100) _    // Okay, here is the '_'
adder2: Int => Int

scala> adder2(1)                    // Now i can call the curried function
res43: Int = 101

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