imp: print: show a disambiguating decimal mark when needed

Eg "1,000" (with , as a thousands separator and no decimal digits) is
now displayed with a decimal mark: "1,000.".

"1 000" (where space is a thousands separator) is less ambiguous,
but we do the same thing (eg "1 000.") for consistency, and also to
help disambiguate when forgetting to quote a numeric commodity symbol
(eg "1234 0" where 1234 is a symbol that should have been in double quotes).

hledger-lib/Hledger/Data/Amount.hs

@@ -517,6 +517,10 @@ showAmount = wbUnpack . showAmountB noColour
 --
 -- * The special "missing" amount is displayed as the empty string. 
 --
+-- * If an amount is showing digit group separators but no decimal places,
+--   we force showing a decimal mark (with nothing after it) to make
+--   it easier to parse correctly.
+--
 showAmountB :: AmountDisplayOpts -> Amount -> WideBuilder
 showAmountB _ Amount{acommodity="AUTO"} = mempty
 showAmountB opts a@Amount{astyle=style} =
@@ -565,6 +569,8 @@ showAmountDebug Amount{..} =
 
 -- | Get a Text Builder for the string representation of the number part of of an amount,
 -- using the display settings from its commodity. Also returns the width of the number.
+-- A special case: if it is showing digit group separators but no decimal places,
+-- show a decimal mark (with nothing after it) to make it easier to parse correctly.
 showamountquantity :: Amount -> WideBuilder
 showamountquantity amt@Amount{astyle=AmountStyle{asdecimalmark=mdec, asdigitgroups=mgrps}} =
     signB <> intB <> fracB
@@ -578,10 +584,14 @@ showamountquantity amt@Amount{astyle=AmountStyle{asdecimalmark=mdec, asdigitgrou
     (intPart, fracPart) = T.splitAt intLen numtxtwithzero
     intB = applyDigitGroupStyle mgrps intLen $ if decplaces == 0 then numtxt else intPart
     signB = if mantissa < 0 then WideBuilder (TB.singleton '-') 1 else mempty
-    fracB = if decplaces > 0
+    fracB = if decplaces > 0 || isshowingdigitgroupseparator
       then WideBuilder (TB.singleton dec <> TB.fromText fracPart) (1 + fromIntegral decplaces)
       else mempty
 
+    isshowingdigitgroupseparator = case mgrps of
+      Just (DigitGroups _ (rightmostgrplen:_)) -> intLen > fromIntegral rightmostgrplen
+      _ -> False
+
 -- | Given an integer as text, and its length, apply the given DigitGroupStyle,
 -- inserting digit group separators between digit groups where appropriate.
 -- Returns a Text builder and the number of digit group separators used.

hledger/test/print/print-style.test

@@ -73,3 +73,17 @@ $ hledger -f- print
 #     (a)    A1.234 @ B3.456 = A1.234 @ B3.456
 # 
 # >=
+
+# 3. When showing digit group marks, print always shows a decimal mark as well,
+# even when no decimal digits are shown.
+<
+decimal-mark .
+2023-01-01
+    (a)    1,000
+
+$ hledger -f- print
+2023-01-01
+    (a)          1,000.
+
+>=
+