Memoizing Y combinator

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open System.Collections.Concurrent
let rec memoFix f =
    let dict = ConcurrentDictionary()
    let rec fn x = dict.GetOrAdd(Some x, lazy (f fn x)).Value
    fn

namespace System

namespace System.Collections

namespace System.Collections.Concurrent

val memoFix : f:(('a -> 'b) -> 'a -> 'b) -> ('a -> 'b)

Full name: Script.memoFix

val f : (('a -> 'b) -> 'a -> 'b)

val dict : ConcurrentDictionary<'a option,Lazy<'b>>

Multiple items
type ConcurrentDictionary<'TKey,'TValue> =
  new : unit -> ConcurrentDictionary<'TKey, 'TValue> + 6 overloads
  member AddOrUpdate : key:'TKey * addValueFactory:Func<'TKey, 'TValue> * updateValueFactory:Func<'TKey, 'TValue, 'TValue> -> 'TValue + 1 overload
  member Clear : unit -> unit
  member ContainsKey : key:'TKey -> bool
  member Count : int
  member GetEnumerator : unit -> IEnumerator<KeyValuePair<'TKey, 'TValue>>
  member GetOrAdd : key:'TKey * valueFactory:Func<'TKey, 'TValue> -> 'TValue + 1 overload
  member IsEmpty : bool
  member Item : 'TKey -> 'TValue with get, set
  member Keys : ICollection<'TKey>
  ...

Full name: System.Collections.Concurrent.ConcurrentDictionary<_,_>

--------------------
ConcurrentDictionary() : unit
ConcurrentDictionary(collection: System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<'TKey,'TValue>>) : unit
ConcurrentDictionary(comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
ConcurrentDictionary(concurrencyLevel: int, capacity: int) : unit
ConcurrentDictionary(collection: System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<'TKey,'TValue>>, comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
ConcurrentDictionary(concurrencyLevel: int, collection: System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<'TKey,'TValue>>, comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
ConcurrentDictionary(concurrencyLevel: int, capacity: int, comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit

val fn : ('a -> 'b)

val x : 'a

ConcurrentDictionary.GetOrAdd(key: 'a option, value: Lazy<'b>) : Lazy<'b>
ConcurrentDictionary.GetOrAdd(key: 'a option, valueFactory: System.Func<'a option,Lazy<'b>>) : Lazy<'b>

union case Option.Some: Value: 'T -> Option<'T>