I notice by chance that scala can infer the type of some method's parameter. But I don't understand the exact rule. Can someone explain me why the test1 method work and why the test2 method does not work
object Test {
def test1[A]: A => A = a => a
def test2[A]: A = a
}
I can't find a good title for my question since I don't understand what is happening in this two lines. Do you have any idea?
def test1[A]: A => A = a => a
|____| |____|
the return type an anonymous function
(a function from A to A) (`a` is a parameter of this function)
def test2[A]: A = a
| |
the return type an unbound value
(A) (i.e. not in scope, a is not declared)
The gotcha is that in the first example a is the parameter of an anonymous function, whereas in the second example a is never declared.
test1 is a method that takes no input and returns a function A => A. The name a is given as the imput parameter of the funtion and the function simple reutrns a, it's input.
test2 is a method that takes no input returns a value of type A. The method is defined to return the variable named a but that variable has never been declared so you get an error. you could redefine the method to be def test2[A](a: A): A = a and it would work, because now a has been declared as a variable of type A, it is the parameter of the method.
