On Sun, 26 Dec 2004, Nicolas George wrote:

> Le sextidi 6 nivôse, an CCXIII, briand@aracnet.com a écrit :
> >       try
> >         (input_line chan), false
> >       with
> >         | End_of_file -> "", true
>
> I would have written that
>
> 	try
> 	  Some (input_line chan)
> 	with
> 	  | End_of_file -> None
>
> but the idea is the same. I find it is an irritating limitation of OCaml
> syntax to have to pack and then unpack all local values in order to uncatch
> exceptions. Something like
>
> 	try
> 	  let line = input_line chan in
> 	  untry
>           loop (line :: rlst)
> 	with
> 	  | End_of_file -> List.rev rlst
>
> This syntax is somewhat awkward: untry is neither a third member of the
> try...with structure, because it must be inside the flow of let...in
> declaration, nor a stand-alone statement, because it must not be allowed
> anywhere outside try...with.
>
> On the contrary, as far as I can see, the semantics is quite simple.

Instead of this (try too large):

let readfile chan =
  let rec loop rlst =
    try
      let line = input_line chan in
      loop (line :: rlst)
    with
	End_of_file -> List.rev rlst in
  loop []

We can write that:

let readfile chan =
  let rec loop rlst =
    (try
       let line = input_line chan in
       fun () -> loop (line :: rlst)
     with
	 End_of_file -> fun () -> List.rev rlst) () in
  loop []


And it seems to be handled efficiently by the compiler (confirmation?).
(and for the desperate, it is not difficult to write a Camlp4 syntax
extension which does this :-)


Martin

--
Martin Jambon, PhD
Researcher in Structural Bioinformatics since the 20th Century
The Burnham Institute http://www.burnham.org
San Diego, California