F# Memoization Within a Class - functional-programming

Question

I am struggling to make Memoization work in F# when the function I want to Memoize is a member of a class.
The Dictionary appears to be cleared every time - and so nothing is actually memoized, the result is always recomputed. The same code but with the key functions outside of a calls of a class works just fine.
open System
module TestMemo
open System
open System.IO
open System.Collections.Generic
let Memoize f =
let dict = Dictionary<_, _>()
fun c ->
let exists, value = dict.TryGetValue c
match exists with
| true -> value
| _ ->
let value = f c
dict.Add(c, value)
value
type MyClass() as this =
let rec AddToOne(x) = if x <= 1 then 1 else x + AddToOne(x-1)
let rec AddToOneSkip(x) = if x <= 1 then 1 else x + AddToOneSkip(x-2)
member this.MemoAddToOne = Memoize AddToOne
member this.MemoAddToOneSkip = Memoize AddToOneSkip
[<EntryPoint>]
let main args =
let x = new MyClass()
for i in 1..100000 do
Console.WriteLine(x.MemoAddToOneSkip(i))
for i in 1..100000 do
Console.WriteLine(x.MemoAddToOne(i))
0

Answer 1

When you write this:
member this.MemoAddToOne = Memoize AddToOne
That's not a "value", but a property. A property in .NET is a pair of functions (get + set), but there are also read-only properties, which have only "get". And that's what you created here. They're basically functions in disguise.
So every time somebody accesses x.MemoAddToOne, they're basically calling a function, so every time the body is executed anew, thus making a new call to Memoize every time.
To avoid this, create the memoizers once and then return them from the property getter:
let memoAddToOne = Memoize AddToOne
member this.MemoAddToOne = memoAddToOne
Or use a shortcut for the same thing:
member val MemoAddToOne = Memoize AddToOne