[m-dev.] try syntax

[Paul Bone]

On Thu, Jan 22, 2009 at 05:10:03PM +1100, Peter Wang wrote:
> Hi,
> 
> Catching exceptions is too cumbersome.  I think we should have built in
> syntax, e.g. (adjust accordingly if the parser is too dumb)
> 
>     try [io(!IO)] (
> 	f(X, Y, Z, !IO)
>     ) then (
>         on_success(X, Y, Z)	% outputs in scope
>     ) else (
>         on_failure
>     ) catch [_ : exc1] (	% zero or more catch blocks
>         on_catch_exc1
>     ) catch [E] (
>         rethrow(exception(E))

Should the above (catch all exceptions) be implicity defined as
rethrowing that exception.  That is.  I want to be able to omit this if
all I'm going to do is rethrow the acception, but I want the ability to
specify it incase I want some non-default behaviour.

>     ) finally (			% optional
>         cleanup
>     )
> 
> io(!IO) selects try_io.
> store(!S) selects try_store.
> all(Solutions) selects try_all.
> Otherwise use try.
> 
> We'd implement it by doing a tranformation to call the underlying
> predicates in the exception module.  The following assumes no `finally'
> block.
> 
>     try_io(
>         (pred({X0, Y0, Z0}::out, !.IO::di, !:IO::uo) is det :-
>             f(X0, Y0, Z0, !IO)
>         ), TryResult, !IO),
>     (
>         TryResult = succeeded({X, Y, Z}),
>         on_success(X, Y, Z)
>     ;
>         TryResult = failed,
>         on_failure
>     ;
>         TryResult = exception(E0),
>         ( univ_to_type(E0, _ : exc1) ->
>             on_catch_exc1
>         ; univ_to_type(E0, E) ->
>             rethrow(exception(E))
>         )
>     )
> 
> Peter

This is a good idea.


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