journal: also parse Ledger-style "virtual posting costs" - (@), (@@)

hledger-lib/Hledger/Read/Common.hs

@@ -735,15 +735,17 @@ simplecommoditysymbolp :: TextParser m CommoditySymbol
 simplecommoditysymbolp = takeWhile1P Nothing (not . isNonsimpleCommodityChar)
 
 priceamountp :: JournalParser m AmountPrice
-priceamountp = (do
+priceamountp = label "transaction price" $ do
+  -- https://www.ledger-cli.org/3.0/doc/ledger3.html#Virtual-posting-costs
+  parenthesised <- option False $ char '(' >> pure True
   char '@'
   priceConstructor <- char '@' *> pure TotalPrice <|> pure UnitPrice
+  when parenthesised $ void $ char ')'
 
   lift (skipMany spacenonewline)
   priceAmount <- amountwithoutpricep -- <?> "unpriced amount (specifying a price)"
 
   pure $ priceConstructor priceAmount
-  ) <?> "price amount"
 
 balanceassertionp :: JournalParser m BalanceAssertion
 balanceassertionp = do
@@ -1363,7 +1365,9 @@ tests_Common = tests "Common" [
             ,astyle=amountstyle{asprecision=0, asdecimalpoint=Nothing}
             }
         }
-    ]
+   ,test "unit price, parenthesised" $ assertParse amountp "$10 (@) €0.5"
+   ,test "total price, parenthesised" $ assertParse amountp "$10 (@@) €0.5"
+   ]
 
   ,let p = lift (numberp Nothing) :: JournalParser IO (Quantity, Int, Maybe Char, Maybe DigitGroupStyle) in
    test "numberp" $ do

hledger-lib/hledger_journal.m4.md

@@ -549,6 +549,12 @@ There are several ways to record a transaction price:
       assets:dollars  $-135          ; for $135
     ```
 
+4.
+5. Like 1 and 2, but the `@` or `@@` is parenthesised; this is for
+   compatibility with Ledger journals 
+   ([Virtual posting costs](https://www.ledger-cli.org/3.0/doc/ledger3.html#Virtual-posting-costs)), 
+   and in hledger is equivalent to 1 and 2.
+
 Use the [`-B/--cost`](hledger.html#reporting-options) flag to convert
 amounts to their transaction price's commodity, if any.
 (mnemonic: "B" is from "cost Basis", as in Ledger).