Curried function in scala

Question

I have a definition of next methods:

def add1(x: Int, y: Int) = x + y

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

the second one is curried version of first one. Then if I want to partially apply second function I have to write val res2 = add2(2) _. Everything is fine. Next I want add1 function to be curried. I write

val curriedAdd = (add1 _).curried

Am I right that curriedAdd is similiar to add2? But when I try to partially apply curriedAdd in a such way val resCurried = curriedAdd(4) _ I get a compilation error. Then I fix it to

val resCurried = curriedAdd(4)

Why the result of a Functions.curried differs from curried version of add function(from add2)?

Answer 1

Firstly curriedAdd is same as add2 _ and not add2. add2 is just a method.

scala> curriedAdd
res52: Int => (Int => Int) = <function1>

scala> add2 _
res53: Int => (Int => Int) = <function1>

About the second question. I think the below is the reason. Doing

scala> val i = curriedAdd(23)
i: Int => Int = <function1>

scala> i _
res54: () => Int => Int = <function0>

scala> curriedAdd(23) _
<console>:10: error: _ must follow method; cannot follow Int => Int
              curriedAdd(23) _

curriedAdd(23) _ does not work. Lets look at scala manual (§6.7)-

The expression e _ is well-formed if e is of method type or if e is a call-by-name parameter. If e is a method with parameters, e _ represents e converted to a function type by eta expansion (§6.26.5). If e is a parameterless method or call-by-name parameter of type =>T , e _ represents the function of type () => T , which evaluates e when it is applied to the empty parameterlist ().

Remember it only evaluates if it is a method or call-by-name parameter. In curriedAdd(23) _, it does not evaluate curriedAdd(23) but checks if it is a method or call-by-name. It is not a method nor a call-by-name parameter.

It is not by-name because by-name is the property of variable. Above you get a by-name parameter after evaluating curriedAdd(23) but curriedAdd(23) in itself is not a by-name variable. Hence the error (Ideally the compiler should have coverted it). Note that the below works:

scala> curriedAdd(23)
res80: Int => Int = <function1>

scala> res80 _
res81: () => Int => Int = <function0>

The above works because res80 _, here you are applying _ to a call-by-name parameter and hence does the conversion.