[m-dev.] Socket Library

[Peter Ross]

On Wed, Oct 20, 2010 at 11:16 AM, Julien Fischer
<juliensf at csse.unimelb.edu.au> wrote:
>
> On Wed, 20 Oct 2010, Paul Bone wrote:
>
>> On Wed, Oct 20, 2010 at 08:39:32AM +1100, Peter Ross wrote:
>>>
>>> On Wed, Oct 20, 2010 at 3:04 AM, Daniel Waterworth
>>> <da.waterworth at gmail.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> I'm making a socket library in mercury. It works (I have sent simple
>>>> HTTP requests using the code I've written and it supports IPv6), but
>>>> as it stands it doesn't check for errors. So, how should C code go
>>>> about raising exceptions? I haven't found any documentation on the
>>>> subject.
>>>>
>>> If I was writing a socket library, I would make sure that it
>>> implements the stream interface from the standard library.  In that
>>> case errors are raised by constructing a mercury type which indicates
>>> the success or failure of the operation.
>>>
>>> Generally I write code like the following to handle that case.  First
>>> you write a low level predicate whose body is written in C.  That
>>> predicate returns an integer indicating whether or not the operation
>>> succeeded and returns a string which is the reason why it failed.
>>>
>>> :- pred send_string_c(string::in, int::out, string::out, io::di, io::uo)
>>> is det.
>>>
>>> The you write you high level interface.
>>>
>>> :- pred send_string(string::in, result::out, io::di, io::uo) is det.
>>>
>>> send_string(S, Result, !IO) :-
>>>  send_string_c(S, Res, Msg, !IO),
>>>  ( Res = -1 ->
>>>    Result = error(Msg)
>>>  ;
>>>    Result = ok
>>>  ).
>
> I would recommend using an exported enumeration to return the result
> status.  Passing small integers back and forth over the foreign language
> interface tends to lead to less maintainable code IMO.
>
I agree with that point, exported enumerations are a nice way to do
that.  They didn't exist when I wrote most of my interfaces.

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