why does performance improve when using `static let` over `static member`?

rasx 0
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Not only did I see an improvement in performance after switching to IDictionary from Map (which is explored in a question from over 10 years ago), I got even more speed when I went from this type:

type MyCollection =

    static member Collection = [
        (1, "one")
        (2, "two")
        ...
    ] |> dict

    static member Get id = MyCollection.Collection[id]

…to a type like this:

type MyCollection() =

    static let collection = [
        (1, "one")
        (2, "two")
        ...
    ] |> dict

    static member Get id = collection[id]

Why does performance improve when using static let over static member?

collections f# .net
1 Answers

You can see the difference if you look at the decompilation in Sharplab

static member Collection compiles to a property with a getter. i.e. it is syntactic sugar for static member Collection() hence when you call MyCollection.Collection it creates a new IDictionary every call.

By contrast static let collection is essentially a standard binding to a value with special auto-initialisation code. It only creates the IDictionary once.

tranquillity0
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