cil - What is the (# ... #) syntax seen in F# standard library implementation?

Question

Reading sources of Array2D module, I've stumbled upon this interesting construct in implementation of many core functions, for example:

[<CompiledName("Get")>]
let get (array: 'T[,]) (n:int) (m:int) =  (# "ldelem.multi 2 !0" type ('T) array n m : 'T #)  

I can only assume that this is the syntax to inline CIL and is used here obviously to gain performance benefits. However, when I've tried to use this syntax in my program, I get an error:

warning FS0042: This construct is deprecated: it is only for use in the F# library

What exactly is this? Is there any detailed documentation?

Answer 1

I think that this has 2 purposes:

1. These functions compile down to exactly 1 CIL instruction which has to be encoded somewhere, so encoding at the source seems best.
2. It allows for some extra trickery with defining polymorphic Add functions in a high performance way which is hard with the F# type system.

You can actually use this but you have to specify the --compiling-fslib (undocumented) and --standalone flags in your code.