Question

This post from F# News states that F# can inline a function passed as an argument.
Is it always the case? Does it happen automatically?

Answer 1

The answer is no. If you have a function like the following, lambdas do not get inlined.
In the following code lambdas are invoked using FSharpFunc<,>.InvokeFast()

let fold f s l = 
    let rec loop acc l =
        match l with []->acc |h::t->loop (f acc h) t
    loop s l

let list = [1;2;3;4]

list|>fold (fun acc x->x+acc) 0|>printfn "%d"
list|>fold (fun acc x->x*acc) 1|>printfn "%d"

But if you mark fold function as inline situation changes. Not only fold function gets inlined, but lambdas get inlined as well. I used .NET Reflector to confirm that.