Comment author: administrator

In the following case it warns for the row | hd::tl ->
'nil' doesn't occur in the reference manual. I would say that this is
a bit dangerous, since most programmers would use say that nil is nil,
i.e. a synononym for [].

let rec foo (l:int list) : unit =
match l with
| nil -> ()
| hd::tl -> foo tl

"nil" is just an identifier like any other. What you wrote is
equivalent to

let rec foo (l:int list) : unit =
match l with
| x -> ()
| hd::tl -> foo tl

or indeed any name instead of "x", so the first line is a "catch-up"
case. With this in mind, the compiler warning should make sense.

• Xavier Leroy

Comment author: administrator

User's misunderstanding.