try showing integral "maxprecision" amounts without the .0 (except in add's defaults, where it helps)

hledger-lib/Hledger/Data/Amount.hs

@@ -53,6 +53,7 @@ module Hledger.Data.Amount (
                             isReallyZeroMixedAmountCost,
                             isZeroMixedAmount,
                             maxprecision,
+                            maxprecisionwithpoint,
                             missingamt,
                             normaliseMixedAmount,
                             nullamt,
@@ -183,13 +184,20 @@ showamountquantity :: Amount -> String
 showamountquantity (Amount (Commodity {decimalpoint=d,precision=p,separator=s,separatorpositions=spos}) q _) =
     punctuatenumber d s spos $ qstr
     where
-    qstr -- | p == maxprecision && isint q = printf "%d" (round q::Integer)
-         | p == maxprecision            = printf "%f" q
-         | otherwise                    = printf ("%."++show p++"f") q
     -- isint n = fromIntegral (round n) == n
+    qstr -- | p == maxprecision && isint q = printf "%d" (round q::Integer)
+         | p == maxprecisionwithpoint    = printf "%f" q
+         | p == maxprecision             = chopdotzero $ printf "%f" q
+         | otherwise                    = printf ("%."++show p++"f") q
+
+chopdotzero str = reverse $ case reverse str of
+                              '0':'.':s -> s
+                              s         -> s
 
 -- | A special precision value meaning show all available digits.
-maxprecision = 999999
+maxprecision = 999998
+-- | Similar, forces display of a decimal point.
+maxprecisionwithpoint = 999999
 
 -- | Replace a number string's decimal point with the specified character,
 -- and add the specified digit group separators.

hledger/Hledger/Cli/Add.hs

@@ -122,8 +122,10 @@ getPostings ctx accept historicalps enteredps = do
                            | n > 1             = Just balancingamountstr
                            | otherwise         = Nothing
               where
-                historicalamountstr = showMixedAmountWithPrecision maxprecision $ pamount $ fromJust bestmatch'
-                balancingamountstr  = showMixedAmountWithPrecision maxprecision $ negate $ sumMixedAmountsPreservingHighestPrecision $ map pamount enteredrealps
+                -- force a decimal point in the output in case there's a
+                -- digit group separator that would be mistaken for one
+                historicalamountstr = showMixedAmountWithPrecision maxprecisionwithpoint $ pamount $ fromJust bestmatch'
+                balancingamountstr  = showMixedAmountWithPrecision maxprecisionwithpoint $ negate $ sumMixedAmountsPreservingHighestPrecision $ map pamount enteredrealps
       amountstr <- askFor (printf "amount  %d" n) defaultamountstr validateamount
       let amount  = fromparse $ runParser (someamount <|> return missingamt) ctx     "" amountstr
           amount' = fromparse $ runParser (someamount <|> return missingamt) nullctx "" amountstr

tests/add.test

@@ -86,7 +86,7 @@ a
 b
 
 .
->>> /a  +£1,000.0/
+>>> /a  +£1,000.00/
 # 9. default amounts should not fail to balance due to precision
 bin/hledger -f nosuch.journal add
 <<<