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	Wed, 25 Apr 2001 20:38:43 -0400 (EDT)
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Date: Wed, 25 Apr 2001 20:38:43 -0400 (EDT)
From: Patrick M Doane <patrick@watson.org>
To: Chris Hecker <checker@d6.com>
cc: caml-list@inria.fr
Subject: Re: [Caml-list] two unrelated questions
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On Wed, 25 Apr 2001, Chris Hecker wrote:

> 
> 1.  What is the right "functional pattern" for early-outing on success
>     while using an iter/map/fold type function?  Say I'm using iter to
>     search for something in an opaque datastructure.  Should I throw
>     an exception to get out, or is that bad style?  I guess this
>     question only makes sense for iter, since map/fold produce results
>     that you theoretically want to preserve.  So, the question is
>     really, given an iter-style interface to a datastructure (one that
>     takes an ('a -> unit)), how do you tell it to stop iterating?  I
>     guess if the function was ('a -> bool) you could do it that way,
>     but most iters aren't ((List|Array|Hashtbl).iter, for example).
>     Is throwing an exception the best bet?
> 

One way to think about exceptions is to treat them simply as control flow
expressions. For example, the 'break' statement is used in C/C++ to
early-out of iteration constructs. This translates naturally to
exceptions:

  exception Break
  try
    List.iter (fun ..  -> ...  raise Break) l
  with Break -> ()

We can even get labelled breaks like Java:

  exception Break of string
  try
    List.iter (fun ... ->
      try
        List.iter (fun ... ->
          if .. then raise (Break "inner")
                else raise (Break "outer")
        ) list1
      with Break "inner" -> ()
    ) list2
  with Break "outer" -> ()

I personally don't think this is bad style but others may have a different
opinion.

It also seems that the overhead for a try block in Caml is relatively low,
but I've never looked too much at the compiled code to see what it is
doing.

Patrick

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