[m-dev.] proposal: drop automatic initialisation of solver types

[Julien Fischer]

On Fri, 26 Oct 2007, Mark Brown wrote:

> On 26-Oct-2007, Mark Brown <mark at csse.unimelb.edu.au> wrote:
>> On 26-Oct-2007, Julien Fischer <juliensf at csse.unimelb.edu.au> wrote:
>>>
>>> There has been an on-and-off discussion about removing support for
>>> automatic
>>> initialisation for solver types.
>>>
>>> Automatic initialisation is problematic because:
>>>
>>> * it prevents solver types from being properly polymorphic, e.g.
>>>   you cannot have the type foo(bar) where:
>>>
>>> 	:- solver type foo(T).
>>> 	:- solver type bar.
>>>
>>> * the introduction of calls to the initialisation predicates complicates
>>>   mode analysis.  Also, the compiler doesn't do a very good job of
>>>   introducing them anyway: the introduction of the calls can change det
>>>   code into semidet code etc.
>>>
>>> * we don't use automatic intialisation in practice anyway.  The solver
>>>   types in G12 all require (or we recommend) using explicit
>>>   initialisation predicates.
>>>
>>> * mode error messages are often poor or misleading because of
>>>   interactions between automatic initialisation and impure code.
>>
>> * Sometimes you don't want to provide =/2, but provide some other mechanism
>>   for constraining variables to be equal.  In this case, it would be very
>>   useful for the compiler to detect implicit attempts to use =/2, and report
>>   them as a mode error.
>
> I meant initialisation not equality -- sometimes there is no sensible
> default (for example when the variables always have bounds, but there is
> no maximum bound).  But I was going to point out that this problem is
> similar for equality, so it might also be nice if the default equality for
> solver types was to throw an exception.

That's actually what I would have expected it to be if you omitted them
(equality and comparison) anyway.

Julien.
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