[m-dev.] Socket Library

[Julien Fischer]

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.

Julien.
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