"inline" keyword not accepted next to "fun" keyword

Repost of issue Functions defined as a value can’t be inlined where Don Syme suggested to post it here.

The following two definitions are semantically identical and compiled to exactly the same code:

let add (x : int) (y : int) : int = 
   x + y

let add : int -> int -> int =
    fun x y -> x + y

Yet the inline keyword is not allowed next to fun while it is allowed next to let.

That is:

let inline add (x : int)  (y : int) : int = 
   x + y

compiles, but neither

let add : int -> int -> int =
    inline fun x y -> x + y

nor

let add : int -> int -> int =
    fun inline x y -> x + y

compiles.

I consider this a bug because

let add : int -> int -> int =
    fun x y -> x + y

is valid syntax for defining a function, but making it inline is not supported. It should not make a difference which style I choose. Especially in terms of maintainability - right now - I would have to rewrite the function to add inline which is a considerable and unnecessary inconvenience.

[drvink]

Just throwing in my two cents: I’d been meaning to file an issue that you couldn’t do “let inline fn = function | …cases…” for a while, and this issue covers that, so I agree it would be a nice thing to have.

[dsyme]

@amazingant @exercitusvir @21c-HK I’m ok with this this adjustment being made to the language (though I don’t particularly favour its use to write F# code in a non-idiomatic style trying to emulate styles of other languages). So I’ve changed the status of the issue on uservoice to “approved” - though I don’t personally plan to work on the adjustment.

I agree with @brodyberg above that the way to address this is for someone to propose a PR. I suggest starting by finding the place where the current error is raised, e.g. by searching FSComp.txt or other files for the error message text.