User defined aggregate

[julienhenry]

Aggregates in Soufflé are very useful. However, there are use-cases where a developer would want to use its own aggregating function (instead of the intrinsics min, max, count, sum, etc.)
In this PR, I propose a way to use a user defined functor, declared as usual with a .functor ..., as the function to apply on an aggregate.

A example use of user-defined aggregate would be :

.type Interval = [lb: number, ub:number]

.functor lub(i1:Interval, i2:Interval):Interval stateful

.decl I(x:number, i:Interval)
.input I

.decl leastUpperBound(x:number, i:Interval)
leastUpperBound(x, lub) :- 
    I(x, _),
    lub = @@lub i : [0,0], {I(x, i)}.

The above example:

• initializes the accumulating variable res to [0,0].
• updates res = @lub(res, i) for each i satisfying I(x,i).

We can discuss on the syntax if the above one is not convenient...

Current limitations:

• only stateful functors can be used
• functors must have exactly two arguments, and the first argument being passed is always the accumulating variable.
• Behavior may not be deterministic if functor is not commutative/associative...

[codecov]

Codecov Report

Merging #2282 (b698a0b) into master (ba99149) will increase coverage by 0.03%.
The diff coverage is 83.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2282      +/-   ##
==========================================
+ Coverage   77.39%   77.43%   +0.03%     
==========================================
  Files         461      468       +7     
  Lines       29004    29162     +158     
==========================================
+ Hits        22448    22581     +133     
- Misses       6556     6581      +25     

Impacted Files	Coverage Δ
src/ast/Aggregator.h	100.00% <ø> (ø)
src/ast/analysis/Functor.h	50.00% <ø> (ø)
src/ast/analysis/typesystem/PolymorphicObjects.h	100.00% <ø> (ø)
src/ast/analysis/typesystem/Type.h	60.00% <ø> (ø)
...c/ast/analysis/typesystem/TypeConstrainsAnalysis.h	100.00% <ø> (ø)
src/ast/transform/InlineRelations.cpp	64.86% <0.00%> (-1.24%)	⬇️
src/ast/transform/MagicSet.cpp	91.05% <0.00%> (-0.89%)	⬇️
src/ast2ram/utility/TranslatorContext.h	100.00% <ø> (ø)
src/interpreter/Engine.h	100.00% <ø> (ø)
src/ram/utility/Visitor.h	54.54% <ø> (ø)
... and 35 more

[b-scholz]

@julienhenry - there is a clash with your changes. Could you fix it?

[julienhenry]

@b-scholz I have updated to latest master branch. We still need to verify in the TypeChecker that the user-defined functor used in the aggregate is used correctly, there is no such check at the moment.
Also, I'm not very happy with the double '@' syntax. A single '@' would look better but causes conflicts in the parser.

[b-scholz]

Thanks for the contribution! Please, can you update the documentation as well?