Allow exceptions to a group of rules

[denismerigoux]

The problem

Suppose you have a piece of legislation that defines a quantity piecewise:

## Article 1

If the person is less than 25 years old, then the benefit is $1000.

```catala
scope Benefit:
  definition benefit under_condition person.age <= 25 consequence equals $1000
```

## Article 2

If the person is more than 25 years old, then the benefit is $500.

```catala
scope Benefit:
  definition benefit under_condition person.age > 25 consequence equals $500
```

Then, another article defines an exceptional case for the benefit:

## Article 3

If the person is disabled, the benefit is equal to $2000.

To formalize that in Catala, you would want the definition in article 3 to be an exception to the group of piecewise definitions in articles 1 and 2. However, you cannot do that currently since you can only declare an exception to a single rule. This situation happens in a real-world example :

catala/examples/allocations_familiales/securite_sociale_L.catala_fr

Lines 47 to 54 in 62e1e01

	champ d'application PrestationsFamiliales :
	# TODO: ajouter le cas d'une ou plusieurs exceptions à un ensemble
	# de définitions par morceaux regroupées par une même étiquette
	# (non encore implémenté)
	étiquette cas_base règle droit_ouvert de enfant sous condition
	enfant.obligation_scolaire sous forme Avant ou
	enfant.obligation_scolaire sous forme Pendant
	conséquence rempli

.

Solution

Syntax

I propose to piggyback on the label syntax and allow to have the same label being applied to several definitions. Using the same label effectively "groups" them together :

scope Benefit:
  label benefits_base_case 
  definition benefit under condition person.age > 25 consequence equals $1000
  
  label benefits_base_case 
  definition benefit under condition person.age <= 25 consequence equals $500
  
  exception benefits_base_case
  definition benefit under condition person.is_disabled consequence equals $2000

Semantics

The semantics effect of this proposal will be limited to how the default tree is built from the various definitions and the exceptions relationships between them. The critical function that will be modified is

catala/compiler/desugared/desugared_to_scope.ml

Lines 25 to 42 in cacf605

	let def_map_to_tree (def_info : Ast.ScopeDef.t) (def : Ast.rule Ast.RuleMap.t) : rule_tree list =
	let exc_graph = Dependency.build_exceptions_graph def def_info in
	Dependency.check_for_exception_cycle exc_graph;
	(* we start by the base cases: they are the vertices which have no successors *)
	let base_cases =
	Dependency.ExceptionsDependencies.fold_vertex
	(fun v base_cases ->
	if Dependency.ExceptionsDependencies.out_degree exc_graph v = 0 then v :: base_cases
	else base_cases)
	exc_graph []
	in
	let rec build_tree (base_case : Ast.RuleName.t) : rule_tree =
	let exceptions = Dependency.ExceptionsDependencies.pred exc_graph base_case in
	match exceptions with
	| [] -> Leaf (Ast.RuleMap.find base_case def)
	| _ -> Node (List.map build_tree exceptions, Ast.RuleMap.find base_case def)
	in
	List.map build_tree base_cases

In the current version, the vertices of the exceptions graph are single rules or definitions. With this proposal, the vertices should be sets of rules. Several rules r_1,...,r_n grouped together shall be encoded with the default < r_1 ... r_n | false :- EmptyError > so that they are at the same level exception-wise.