Re: [Caml-list] lazy vs fun

[Martin Jambon <martin.jambon@ens-lyon.org>]

Martin Jambon wrote:
> Stéphane Glondu wrote:
>> Warren Harris a écrit :
>>> Is there any advantage to using lazy evaluation in ocaml rather than
>>> just using thunks to defer evaluation? [...]
>> Two things I can think of right now: they are evaluated only once (even
>> if you call Lazy.force several times), and you can do pattern matching
>> with them.
> 
> 
> Note that the memoization feature can be implemented like this:
> 
> let lz f =
>   let result = ref `None in
>   fun () ->
>     match !result with
>         `None ->
>           (try
> 	     let y = f () in
>              result := `Result y;
> 	     y
>            with e ->
> 	     result := `Exn e;
> 	     raise e
> 	  )
>       | `Result y -> y
>       | `Exn e -> raise e


Oops.
The following makes it possible for f to be garbage-collected:


let lz f =
  let result = ref (`None f) in
  fun () ->
    match !result with
        `None f ->
          (try
	     let y = f () in
             result := `Result y;
	     y
           with e ->
	     result := `Exn e;
	     raise e
	  )
      | `Result y -> y
      | `Exn e -> raise e



Martin

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