On Fri, Jun 06, 2003 at 01:49:55PM -0500, Brian Hurt wrote:
> The problem with try/with is that it's way to easy to break tail recursion 
> using try/with.  About every other week someone comes to the Ocaml 
> beginners list where they are doing something like:
> 
> let rec read_all_lines chan accum =
>    try
>        let line = input_line chan in
>        read_all_lines chan (line :: accum)
>    with
>       End_of_file -> List.rev accum

This is a good argument for the alternative exception handling syntax
proposed by in "Exceptional syntax", JFP 11(4):395-410,
<http://research.microsoft.com/~nick/except.ps>. They suggest a form

try x <= e1 in e2 unless E => e3

which acts like "let x = e1 in e2" when e1 causes no exceptions, but
otherwise goes directly to the handler and skips e2 entirely.

So the function above could be written:

let rec read_all_lines chan accum =
   try
       line <= input_line chan 
   in
       read_all_lines chan (line :: accum)
   unless
       End_of_file => List.rev accum

It looks almost the same, but here the scope of the handler does _not_
extend over the recursive call, only the binding

Actually, as pointed out by the article, this thing has been suggested
on caml-list before: <http://caml.inria.fr/archives/199912/msg00078.html>.


Lauri Alanko
la@iki.fi

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