Re: [Caml-list] Re: '_a

[Christophe TROESTLER <Christophe.Troestler@umh.ac.be>]

On 28 Jan 2005, skaller <skaller@users.sourceforge.net> wrote:
> 
> let x = 
> 	let rec f l -> | [] -> raise Not_found 
> 	| h :: t -> if h == v then (raise Found; 0) else f t
> 	in try f l with Found -> 1 | Not_found -> 2
> in print_endline (string_of_int x)
> 
> where the compiler doesn't know f l cannot return, so it needs a
> useless '0' after the Found case to get the typing correct.

Not quite,

let find v l =
  let x = 
    let rec f = function
      | [] -> raise Not_found 
      | h :: t -> if h = v then raise Found else f t
    in try f l with Found -> 1 | Not_found -> 2
  in print_endline (string_of_int x)

has type "val find : 'a -> 'a list -> unit = <fun>".  (BTW, note that
the equality is "=" in Caml.)  Maybe you mean something like

let cl file =
  let fh = open_in file in
  let nl = ref 0 in
  try
    while true do
      let _ = input_line fh in
      incr nl
    done
  with End_of_file -> !nl

where the [while] has type [unit] while [!nl] has type [int] and the
two cannot be unified.  But this is because there is no way for the
compiler to know that a loop indeed never ends, so one has to tell:

let cl file =
  let fh = open_in file in
  let nl = ref 0 in
  try
    while true do
      let _ = input_line fh in
      incr nl
    done;
    assert false
  with End_of_file -> !nl

With that everything is fine.

Regards,
ChriS