csv: handle more sign variations, eg a sign by itself

simplifySign now covers a few more sign combinations that might arise.
And in particular, it strips a standalone sign with no number,
which simplifies sign flipping with amount-in/amount-out.

hledger-lib/Hledger/Read/CsvReader.hs

@@ -1149,11 +1149,18 @@ unknownIncomeAccount  = "income:unknown"
 type CsvAmountString = Text
 
 -- | Canonicalise the sign in a CSV amount string.
--- Such strings can have a minus sign, negating parentheses,
--- or any two of these (which cancels out).
+-- Such strings can have a minus sign, parentheses (equivalent to minus),
+-- or any two of these (which cancel out),
+-- or a plus sign (which is removed),
+-- or any sign by itself with no following number (which is removed).
+-- See hledger > CSV FORMAT > Tips > Setting amounts.
+--
+-- These are supported (note, not every possibile combination):
 --
 -- >>> simplifySign "1"
 -- "1"
+-- >>> simplifySign "+1"
+-- "1"
 -- >>> simplifySign "-1"
 -- "-1"
 -- >>> simplifySign "(1)"
@@ -1162,15 +1169,26 @@ type CsvAmountString = Text
 -- "1"
 -- >>> simplifySign "-(1)"
 -- "1"
+-- >>> simplifySign "-+1"
+-- "-1"
 -- >>> simplifySign "(-1)"
 -- "1"
 -- >>> simplifySign "((1))"
 -- "1"
+-- >>> simplifySign "-"
+-- ""
+-- >>> simplifySign "()"
+-- ""
+-- >>> simplifySign "+"
+-- ""
 simplifySign :: CsvAmountString -> CsvAmountString
 simplifySign amtstr
   | Just ('(',t) <- T.uncons amtstr, Just (amt,')') <- T.unsnoc t = simplifySign $ negateStr amt
   | Just ('-',b) <- T.uncons amtstr, Just ('(',t) <- T.uncons b, Just (amt,')') <- T.unsnoc t = simplifySign amt
   | Just ('-',m) <- T.uncons amtstr, Just ('-',amt) <- T.uncons m = amt
+  | Just ('-',m) <- T.uncons amtstr, Just ('+',amt) <- T.uncons m = negateStr amt
+  | amtstr `elem` ["-","+","()"] = ""
+  | Just ('+',amt) <- T.uncons amtstr = simplifySign amt
   | otherwise = amtstr
 
 negateStr :: Text -> Text

hledger/hledger.m4.md

@@ -3936,11 +3936,10 @@ Here are the ways to set a posting's amount:
    guessing an appropriate sign.
 
    - **If hledger guesses the wrong sign:**\
-     You can prepend a minus sign to flip it. But, do this only for non-empty values (see below). Eg:
+     You can prepend a minus sign to flip it. Eg:
 
       ```rules
-      # flip amount-out's sign, if it is non-empty:
-      if %amount-out .
+      fields date, description, amount-in, amount-out
       amount-out -%amount-out
       ```
 
@@ -3984,7 +3983,7 @@ Here are the ways to set a posting's amount:
       account1 assets:checking
       ```
 
-There is some automatic special handling for an amount's sign, for convenience:
+There is some special handling for an amount's sign, to simplify parsing and sign-flipping:
 
 - **If an amount value begins with a plus sign:**\
   that will be removed: `+AMT` becomes `AMT`
@@ -3992,8 +3991,11 @@ There is some automatic special handling for an amount's sign, for convenience:
 - **If an amount value is parenthesised:**\
   it will be de-parenthesised and sign-flipped: `(AMT)` becomes `-AMT`
 
-- **If an amount value begins with two minus signs:**\
-  they cancel out and will be removed: `--AMT` becomes `AMT`
+- **If an amount value has two minus signs (or two sets of parentheses, or a minus sign and parentheses):**\
+  they cancel out and will be removed: `--AMT` or `-(AMT)` becomes `AMT`
+
+- **If an amount value contains just a sign (or just a set of parentheses):**\
+  that is removed, making it an empty value. `"+"` or `"-"` or `"()"` becomes `""`.
 
 ### Setting currency/commodity