[m-dev.] shims

[Mark Brown]

On Mon, Sep 15, 2014 at 4:23 PM, Mark Brown <mark at mercurylang.org> wrote:
> On Mon, Sep 15, 2014 at 3:46 PM, Peter Wang <novalazy at gmail.com> wrote:
>> Hi Mark,
>>
>> On Sun, 14 Sep 2014 12:51:44 +1000, Mark Brown <mark at mercurylang.org> wrote:
>>> This is much more verbose than Peter's suggestion, but aside from the
>>> better checking it's also much more flexible because you can target
>>> different libraries, not just different versions of the same library.
>>> For example, if there is a module called fastlib that only some
>>> developers have in their path, you might write:
>>>
>>>     :- pred algorithm(int::in, int::out) is det,
>>>     :- if.
>>>         :- import_module fastlib.           % This can fail.
>>>         :- pred fastlib.algorithm(int::in, int::out) is det.
>>>     :- then.
>>>         algorithm(X, Y) :- fastlib.algorithm(X, Y).
>>>     :- else.
>>>         algorithm(X, Y) :- slow_algorithm(X, Y).
>>>     :- end_if.
>>>
>>> Note that, even though fastlib is imported, we still have to declare
>>> the predicates from it that are used. This enables the then branch to
>>> be checked even if the import fails, and also allows a check that
>>> fastlib itself is the right version.
>>
>> I like it.
>>
>> Is it fair to think of each if-then branch declaring something like an
>> anonymous nested sub-module, with forwarding of predicates and types
>> from the parent module to the sub-module?
>
> Exactly. That was the motivation for the syntax, too.

For good measure, here is an alternative syntax. This time the
branches are written in the style of nested sub-modules, but the
conditions are written in the style of typeclass definitions. I think
I prefer this, because satisfying these conditions is quite like
satisfying the conditions of a typeclass.

:- pred algorithm(int::in, int::out) is det.

:- if [
        import_module fastlib,
        pred fastlib.algorithm(int::in, int::out) is det
    ]

algorithm(X, Y) :- fastlib.algorithm(X, Y).

:- else.

algorithm(X, Y) :- slow_algorithm(X, Y).

:- end_if.

Cheers,
Mark.