[m-rev.] Xlib interface for extras

[Ian MacLarty]

On 24 Sep 2004, at 10:01, Ralph Becket wrote:

> Ian MacLarty, Thursday, 23 September 2004:
>>>
>>> That would be a pain.  The xlib module is supposed to be as simple as
>>> possible.
>>>
>>
>> The extras are one of the first places a new Mercury user will look to
>> see how best to implement something, so I think we should be setting a
>> good example with the code we put in extras.  I don't think that 
>> making
>> things that do IO impure to hide the IO state threading is the best 
>> way
>> to do things.  Nor do I think it makes the code any more readable or
>> writable.  In fact threading IO state through calls requires less
>> typing that making an impure call.
>
> The problem is in reporting failure.  I can't use IO states if I want a
> predicate to simply fail in this case.  Alternatively I could export a
> bunch of procedures for returning maybe(T) values, one for each T that 
> I
> need to be able to return.  Or I could call a polymorphic Mercury
> predicate to do the job, but that gets complicated because then I have
> to obtain the corresponding type_infos.
>

The majority of impure preds seem to be det, so making these pure 
should be quite easy. From what I can see there are only 6 types output 
from impure semidet preds (display_ptr, drawable, gc, color_ptr, 
font_struct_ptr and event_ptr), so you'd only need to export 6 
monomorphic procs to return the maybe types.

I still think it'd be better to publish pure code where possible as an 
example of best practice to anyone who may be looking at the code for 
guidance on how to implement a pure interface to a foreign API.

Ian.

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