Support [<InlineIfLambda>]

[MangelMaxime]

Description

If I understand correctly this news: https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-6#inlineiflambda

This code:

let inline iterateTwice ([<InlineIfLambda>] action) (array: 'T[]) =
    for j = 0 to array.Length-1 do
        action array[j]
    for j = 0 to array.Length-1 do
        action array[j]

let arr = [| 1.. 100 |]
let mutable sum = 0
arr  |> iterateTwice (fun x ->
    sum <- sum + x)

should be equivalent to

let arr = [| 1.. 100 |]
let mutable sum = 0
for j = 0 to arr.Length-1 do
    sum <- sum + arr[j]
for j = 0 to arr.Length-1 do
    sum <- sum + arr[j]

However, when looking at the generated JS it seems like [<InlineIfLambda>] does nothing:

export const arr = toArray(rangeDouble(1, 1, 100));

export let sum = createAtom(0);

(function () {
    const action_1 = (x) => {
        sum(sum() + x);
    };
    const array_1 = arr;
    for (let j = 0; j <= (array_1.length - 1); j++) {
        action_1(array_1[j]);
    }
    for (let j_1 = 0; j_1 <= (array_1.length - 1); j_1++) {
        action_1(array_1[j_1]);
    }
})();

Expected and actual results

Should generates something like that I think:

export const arr = toArray(rangeDouble(1, 1, 100));

export let sum = createAtom(0);

for (let j = 0; j <= (arr.length - 1); j++) {
    sum(sum() + arr[j]);
}

for (let j = 0; j <= (arr.length - 1); j++) {
    sum(sum() + arr[j]);
}

Related information

• Fable version: 4.0.6
• Operating system: Linux

[MangelMaxime]

To give a bit of context:

[<InlineIfLambda>] is useful when using CEs for building a DSL and it can greatly improve memory and performance usage from what I understood

I am trying to work on a CE DSL for JSX and I feel like if [<InlineIfLambda>] was working then I could have a DSL with almost zero cost in term of performance compared to JavaScript.

[alfonsogarciacaro]

I think it's possible, although I'm not sure how much work it will involve. This would require "conditional" inlines and check the argument to decide if the function must be inlined or not.

In any case, be careful when using CEs to design a DSL. I've received reports that when used in a large project compilation starts to get really slow, see: dotnet/fsharp#14429

[MangelMaxime]

Rah that's really unfortunate ... thank you for letting me know of this problem.

I thin I will still explore that direction but probably keep it as a POC depending on the situation.