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From: oleg@okmij.org
To: caml-list@inria.fr
In-reply-to: <02dd01cf13ed$9f391950$ddab4bf0$@ffconsultancy.com>
Subject: Re: [Caml-list] How much optimized is the 'a option type ?


> let rec iter_stream f s =
>     match (try Some (MyStream.get s) with End_of_stream -> None) with
>     | None -> ()
>     | Some (x, s') ->
>         f x;
>         iter_stream f s'
>
> where an option is used to keep the function tail-rec (I've heard
> several people tell me they often need to use this), or other cases
> like optional parameters (which are not going to move to Either),
> would be useful and future-proof. 

I concur this is a very common pattern. The immediate thought is that
perhaps functions like MyStream.get should return an (('result,'error)
either) value rather than throw an exception. But there is a better
solution, described in

        Exceptional Syntax 
        Nick Benton and Andrew Kennedy. 
        In Journal of Functional Programming, 11(4):
        395-410, July 2001.
        http://research.microsoft.com/en-us/um/people/akenn/sml/ExceptionalSyntax.pdf

Incidentally, the example presented in Sec 3 of the paper is almost
exactly like the example mentioned on this thread. It is a common
problem, and luckily a good solution has been designed and well
explained. I guess what's lacking is time to implement it.