Odd issue with generic function in Scala - scala

Question

I'm in the process of learning a bit about Scala in order to see how it compares to F#, which is what I spend most of my time with. I'm attempting a pretty straightforward memoization technique for functions. In F# I would use something like this:
open System.Collections.Generic
let memoize (f : 'a -> 'b) =
// A dictionary in which to cache results of previous
// calls of the function
let dict = new Dictionary<'a, 'b>()
let memoizedFunction input =
match dict.TryGetValue(input) with
| true, x -> x
| false, _ ->
let answer = f input
dict.Add(input, answer)
answer
memoizedFunction
In attempting to produce an equivalent version of this in Scala I've come up with the following:
def memoize[A, B](f: A => B): A => B = {
val dict = collection.mutable.WeakHashMap[A, B]()
def memoizedFunction[A, B](input: A): B =
dict.get(input) match {
case x: Some[B] => x
case _ =>
val answer = f(input)
dict += ((input, answer))
answer
}
memoizedFunction _
}
I'm receiving a few errors in the REPL when I attempt to define this function:
<console>:11: error: type mismatch;
found : input.type (with underlying type A)
required: A
dict.get(input) match {
^
<console>:14: error: type mismatch;
found : input.type (with underlying type A)
required: A
val answer = f(input)
^
<console>:15: error: type mismatch;
found : input.type (with underlying type A)
required: A
dict += ((input, answer))
^
Can anyone point out where I'm going wrong here?

Answer 1

The type parameters on memoizedFunction shadow the outer type parameters. The nitty-gritty of Scala generics escapes me, but that's why input's type of A isn't the A that Scala is looking for.
Also, x: Some[B] conflicts with the return type of memoizedFunction, which is B. The idiomatic way to match Option is Some(x) => // do something with x.
So here's what I've got with those 2 things corrected:
def memoize[A, B](f: A => B): A => B = {
val dict = collection.mutable.WeakHashMap[A, B]()
def memoizedFunction(input: A): B =
dict.get(input) match {
case Some(x) => x
case _ =>
val answer = f(input)
dict += ((input, answer))
answer
}
memoizedFunction _
}