scala anonymous function missing parameter type error

Question

I wrote the following

def mapFun[T, U](xs: List[T], f: T => U): List[U] = (xs foldRight List[U]())( f(_)::_ )

and when I did

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Answer 1

It seems to me that because "f" is in the same parameter list as "xs", you're required to give some information regarding the type of x so that the compiler can solve it.

In your case, this will work:

mapFun(List(1,2,3) , (x: Int) => x * x)  

Do you see how I'm informing the compiler that x is an Int?

A "trick" that you can do is currying f. If you don't know what currying is check this out: http://www.codecommit.com/blog/scala/function-currying-in-scala

You will end up with a mapFun like this:

def mapFun[T, U](xs: List[T])(f: T => U): List[U] = 
    (xs foldRight List[U]())( f(_)::_ )

And this will work:

mapFun(List(1,2,3))(x => x * x)

In the last call, the type of x is resolved when the compiler checks the first parameter list.

EDIT:

As Dominic pointed out, you could tell the compiler what your types are. Leading to:

mapFun[Int, Int](List(1,2,3), x => x * x)

Cheers!