How can ConditionalWeakTable be used with multiple references?

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