From dbe29599c2e9bf49368c7a92fe00259aa9cbbe15 Mon Sep 17 00:00:00 2001
From: Hugo Herbelin
Date: Fri, 8 Apr 2016 12:07:57 +0200
Subject: A heuristic to add parentheses in the presence of rules such as

Notation "## c" := (S c) (at level 0, c at level 100).

which break the stratification of precedences. This works for the case
of infix or suffix operators which occur in only one grammar rule,
such as +, *, etc. This solves the "constr" part of #3709, even though
this example is artificial.

The fix is not complete. It puts extra parenthesese even when it is
end of sentence, as in

Notation "# c % d" := (c+d) (at level 3).
Check fun x => # ## x % ## (x * 2).
(* fun x : nat => # ## x % (## x * 2) *)

The fix could be improved by not always using 100 for the printing
level of "## c", but 100 only when not the end of the sentence.

The fix does not solve the general problem with symbols occurring in
more than one rule, as e.g. in:

Notation "# c % d" := (c+d) (at level 1).
Notation "## c" := (S c) (at level 0, c at level 5).
Check fun x => # ## x % 0.
(* Parentheses are necessary only if "0 % 0" is also parsable *)

I don't see in this case what better approach to follow than
restarting the parser to check reversibility of the printing.
---
 interp/notation.ml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'interp/notation.ml')

diff --git a/interp/notation.ml b/interp/notation.ml
index b8f4f32017..1a2e7074fd 100644
--- a/interp/notation.ml
+++ b/interp/notation.ml
@@ -965,7 +965,7 @@ let pr_visibility prglob = function
 (**********************************************************************)
 (* Mapping notations to concrete syntax *)
 
-type unparsing_rule = unparsing list * precedence
+type unparsing_rule = unparsing list * precedence * tolerability option
 type extra_unparsing_rules = (string * string) list
 (* Concrete syntax for symbolic-extension table *)
 let printing_rules =
-- 
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