[m-dev.] `highly ambiguous overloading' ??

[Fergus Henderson]

On 16-Mar-2001, Nancy Mazur <Nancy.Mazur at cs.kuleuven.ac.be> wrote:
> We're trying to figure out the meaning of the next warning: 
> 
> hlds_pred.m:1918: In clause for predicate `hlds_pred:proc_info_create/12':
> hlds_pred.m:1918:   warning: highly ambiguous overloading. This may cause
> hlds_pred.m:1918:   type-checking to be very slow. It may also make your code
> hlds_pred.m:1918:   difficult to understand.
> 
> I've just added an additional field to the proc_info type, and got
> that warning.
> 
> What's wrong here? 
...
> proc_info_create(VarSet, VarTypes, HeadVars, HeadModes, InstVarSet, Detism,
> 		Goal, Context, TVarMap, TCVarsMap, IsAddressTaken, ProcInfo) :-
> 	map__init(StackSlots),
> 	set__init(Liveness),
> 	MaybeHeadLives = no,
> 	RLExprn = no,
> 	Alias = no,
> 	GlobalUse = no, 
> 	Reuse = no,
> 	StaticTerms = no, 	% <========== line 1918 ====================
> 	ModeErrors = [],
> 	ProcInfo = procedure(VarSet, VarTypes, HeadVars, HeadVars, HeadModes,
> 		ModeErrors, InstVarSet,
> 		MaybeHeadLives, Goal, Context, StackSlots,
> 		yes(Detism), Detism, yes, [], Liveness, TVarMap, TCVarsMap,
> 		eval_normal, no, no, no, IsAddressTaken, RLExprn,
> 		Alias, GlobalUse, Reuse, StaticTerms, no, no).

The compiler type-checks things left-to-right, and does breadth-first search
to resolve ambiguities.  If you have a whole bunch of unifications with an
overloaded name like `no' (which is used both in `bool' and in `maybe(T)'), and
nothing to resolve the ambiguity, then the current implementation uses up 2^N
time and space.  For N > 5 you will get a warning.  Here N = 6.

A solution is to write the unification `ProcInfo = procedure(...)'
before all the unifications with `no'; that way the compiler
will be able to use the type of `procedure(...)' to resolve
the ambiguity.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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