ConditionalWeakTable represents a thread-safe way to attach references to other references and not have to worry about garbage collection of the attached references. However, it can only be used with a single reference as a key. How can I use multiple references as keys?
For example, I have a function that accepts three reference types as input and produces a value, and I want to memoize this function based on the references of the three inputs. How can I use ConditionalWeakTable for this?
You can achieve the desired functionality by nesting ConditionalWeakTable:
module Memoize =
/// Memoizes the specified function using reference equality on the input arguments.
/// The result is cached for the lifetime of the keys.
///
/// Don't call with additional arguments as ad-hoc tuples or records,
/// since these will never be reference equal.
let refEq3 (f: 'k1 -> 'k2 -> 'k3 -> 'v) =
let cache = ConditionalWeakTable<'k1, ConditionalWeakTable<'k2, ConditionalWeakTable<'k3, 'v>>>()
fun k1 k2 k3 ->
let inner1 = cache.GetOrCreateValue(k1)
let inner2 = inner1.GetOrCreateValue(k2)
match inner2.TryGetValue(k3) with
| true, v -> v
| false, _ ->
let v = f k1 k2 k3
inner2.TryAdd(k3, v) |> ignore<bool>
v
For completeness, here's a version that uses boxing/unboxing to also allow struct values. Boxing has some performance impact, but in the case of memoization, it is likely that the function you are memoizing has a significantly higher performance impact anyway.
let refEq3 (f: 'k1 -> 'k2 -> 'k3 -> 'v) =
let cache =
ConditionalWeakTable<'k1, ConditionalWeakTable<'k2, ConditionalWeakTable<'k3, obj>>>()
fun k1 k2 k3 ->
let inner1 = cache.GetOrCreateValue(k1)
let inner2 = inner1.GetOrCreateValue(k2)
match inner2.TryGetValue(k3) with
| true, v -> unbox<'v> v
| false, _ ->
let v = f k1 k2 k3
inner2.TryAdd(k3, box v) |> ignore<bool>
v