imp: disallow date: in expr: OR expressions, avoiding unclear semantics [#2177][#2178]

hledger-lib/Hledger/Query.hs

@@ -324,10 +324,12 @@ parseQueryTerm d s = parseQueryTerm d $ defaultprefix<>":"<>s
 -- prefix-operator "NOT e" is always parsed before "e AND e", "e AND e" before "e OR e",
 -- and "e OR e" before "e e".
 --
--- The space-separation operator is left as it was the default before the introduction of
--- boolean operators. It takes the behaviour defined in the interpretQueryList function,
--- whereas the NOT, OR, and AND operators simply wrap a list of queries with the associated
---
+-- The "space" operator still works as it did before the introduction of boolean operators:
+-- it combines terms according to their types, using parseQueryList.
+-- Whereas the new NOT, OR, and AND operators work uniformly for all term types.
+-- There is an exception: queries being OR'd may not specify a date period,
+-- because that can produce multiple, possibly disjoint, report periods and result sets,
+-- and we don't have report semantics worked out for it yet. (#2178)
 --
 -- The result of this function is either an error encountered during parsing of the
 -- expression or the combined query and query options.
@@ -361,8 +363,14 @@ parseBooleanQuery d t =
               _      -> (simplifyQuery $ f qs, qoptss')
 
         -- Containing query expressions separated by "or".
+        -- If there's more than one, make sure none contains a "date:".
         orExprsP :: SimpleTextParser (Query, [QueryOpt])
-        orExprsP = combineWith Or <$> andExprsP `sepBy` (try $ skipNonNewlineSpaces >> string' "or" >> skipNonNewlineSpaces1)
+        orExprsP = do
+          exprs <- andExprsP `sepBy` (try $ skipNonNewlineSpaces >> string' "or" >> skipNonNewlineSpaces1)
+          if ( length exprs > 1
+            && (any (/=Any) $ map (filterQuery queryIsDateOrDate2 . fst) exprs))
+          then fail "sorry, using date: in OR expressions is not supported."
+          else return $ combineWith Or exprs
 
           where
             -- Containing query expressions separated by "and".

hledger/hledger.m4.md

@@ -5082,22 +5082,31 @@ The [print](#print) command is a little different, showing transactions which:
 - match all the other terms.
 
 We also support more complex boolean queries with the `expr:` prefix.
-This allows one to combine queries using `AND`, `OR`, and `NOT`.
-(`NOT` is equivalent to the `not:` prefix.) Some examples:
+This allows one to combine query terms using `and`, `or`, `not` keywords (case insensitive),
+and to group them by enclosing in parentheses.
 
-- Match transactions with 'cool' in the description AND with the 'A' tag
+Some examples:
 
-  `expr:"desc:cool AND tag:A"`
+- Exclude account names containing 'food':
 
-- Match transactions NOT to the 'expenses:food' account OR with the 'A' tag
+  `expr:"not food"`  (`not:food` is equivalent)
 
-  `expr:"NOT expenses:food OR tag:A"`
+- Match things which have 'cool' in the description and the 'A' tag:
 
-- Match transactions NOT involving the 'expenses:food' account OR 
-  with the 'A' tag AND involving the 'expenses:drink' account.
-  (the AND is implicitly added by space-separation, following the rules above)
+  `expr:"desc:cool and tag:A"`  (`expr:"desc:cool tag:A"` is equivalent)
+
+- Match things which either do not reference the 'expenses:food' account, or do have the 'A' tag:
+
+  `expr:"not expenses:food or tag:A"`
+
+- Match things which either do not reference the 'expenses:food' account,
+  or which reference the 'expenses:drink' account and also have the 'A' tag:
+
+  `expr:"expenses:food or (expenses:drink and tag:A)"`
+
+`expr:` has a restriction: `date:` queries may not be used inside `or` expressions.
+That would allow disjoint report periods or disjoint result sets, with unclear semantics for our reports.
 
-  `expr:"expenses:food OR (tag:A expenses:drink)"`
 
 ## Queries and command options
 

hledger/test/query-expr.test

@@ -147,14 +147,13 @@ $ hledger -f - print expr:"not (tag:transactiontag=B)"
 
 >=
 
-# ** 11. Posting-based reports handle OR'd open-ended date periods properly. (#2177)
-<
-2023-12-26 2023
-  (2023)   2023
+# ** 11. With expr:, it's possible for a query (with OR) to specify multiple different date periods. 
+# This is problematic for report semantics in several ways. For example,
+# expr:'(date:2023 AND drinks) OR (date:2024 AND food)' produces two disjoint result sets, and
+# expr:'date:feb or date:may or date:nov' produces three disjoint report periods with holes between them.
+# Can all of our reports handle holes properly, calculate historical starting balances properly, etc ?
+# For now the answer is no and therefore OR-ing of date periods must be disallowed. (#2178)
+$ hledger -f- reg expr:'date:2023 OR date:2024'
+>2 /using date: in OR expressions is not supported/
+>=1
 
-2024-01-26 2024
-  (2024)   2024
-
-$ hledger -f- reg -w80 expr:'date:2023 or date:2024'
-2023-12-26 2023                 (2023)                        2023          2023
-2024-01-26 2024                 (2024)                        2024          4047