Lookups

The examples run inside an active graph.

createLookup f affected source computes f state key for each key you read with Get. Unlike a projection, a lookup has no fixed key set: it creates cells for requested keys. For the common case of selection membership, use a selector. When the source changes, it recomputes live cells for the keys returned by affected prev next.

let stock = createSignal (Map [ "apples", 3; "pears", 0 ])

let changedKeys (prev: Map<string, int>) (next: Map<string, int>) =
    Seq.append prev.Keys next.Keys
    |> Seq.distinct
    |> Seq.filter (fun k -> prev.TryFind k <> next.TryFind k)

let onHand =
    createLookup (fun (m: Map<string, int>) k -> m.TryFind k |> Option.defaultValue 0) changedKeys (fun () -> stock.Value)

onHand.Get "apples", onHand.Get "plums"

(3, 0)

Change the apple count while the pear reader stays quiet. The lookup's affected function names only the key whose count changed.