lib: more compact show instance for Amounts (#812)

Amount's default show instance hid important details, making eg test
failures hard to understand. Showing full detail required increasing
the debug level which was inconvenient.

Now it has a single show instance which shows more information, is
fairly compact, and is pretty-printable with pretty-show.
Ellipses (..) in the output indicate where fields are
- not shown in full detail, and/or
- shown in pseudo syntax (double quoted) to work with pretty-show.

ghci> usd 1
OLD:
Amount {acommodity="$", aquantity=1.00, ..}
NEW:
Amount {acommodity = "$", aquantity = 1.00, aprice = NoPrice, astyle = AmountStyle "L False 2 Just '.' Nothing..", amultiplier = False}

MixedAmount's show instance is unchanged, but showMixedAmountDebug
is affected by this change:

ghci> putStrLn $ showMixedAmountDebug $ Mixed [usd 1]
OLD:
Mixed [Amount {acommodity="$", aquantity=1.00, aprice=, astyle=AmountStyle {ascommodityside = L, ascommodityspaced = False, asprecision = 2, asdecimalpoint = Just '.', asdigitgroups = Nothing}}]
NEW:
Mixed [Amount {acommodity="$", aquantity=1.00, aprice=, astyle=AmountStyle "L False 2 Just '.' Nothing.."}]

hledger-lib/Hledger/Data/Amount.hs

@@ -150,14 +150,12 @@ amountstyle = AmountStyle L False 0 (Just '.') Nothing
 -------------------------------------------------------------------------------
 -- Amount
 
-instance Show Amount where
-  show _a@Amount{..}
-    --  debugLevel < 2 = showAmountWithoutPrice a
-    --  debugLevel < 3 = showAmount a
-    | debugLevel < 6 =
-       printf "Amount {acommodity=%s, aquantity=%s, ..}" (show acommodity) (show aquantity)
-    | otherwise      = --showAmountDebug a
-       printf "Amount {acommodity=%s, aquantity=%s, aprice=%s, astyle=%s}" (show acommodity) (show aquantity) (showPriceDebug aprice) (show astyle)
+instance Show Price where
+  show NoPrice        = "NoPrice"
+  show (UnitPrice a)  = "\"@ " ++ showAmountWithoutPrice a ++ "..\"" 
+  show (TotalPrice a) = "\"@@ " ++ showAmountWithoutPrice a ++ "..\""
+
+deriving instance Show Amount 
 
 instance Num Amount where
     abs a@Amount{aquantity=q}    = a{aquantity=abs q}

hledger-lib/Hledger/Data/Types.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, DeriveGeneric, TypeSynonymInstances, FlexibleInstances, OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards #-}
 {-|
 
 Most data types are defined here to avoid import cycles.
@@ -32,6 +33,7 @@ import Data.Text (Text)
 import Data.Time.Calendar
 import Data.Time.LocalTime
 import System.Time (ClockTime(..))
+import Text.Printf
 
 import Hledger.Utils.Regex
 
@@ -137,10 +139,19 @@ data AmountStyle = AmountStyle {
       asprecision       :: !Int,                 -- ^ number of digits displayed after the decimal point
       asdecimalpoint    :: Maybe Char,           -- ^ character used as decimal point: period or comma. Nothing means "unspecified, use default"
       asdigitgroups     :: Maybe DigitGroupStyle -- ^ style for displaying digit groups, if any
-} deriving (Eq,Ord,Read,Show,Typeable,Data,Generic)
+} deriving (Eq,Ord,Read,Typeable,Data,Generic)
 
 instance NFData AmountStyle
 
+instance Show AmountStyle where
+  show AmountStyle{..} =
+    printf "AmountStyle \"%s %s %s %s %s..\""
+    (show ascommodityside)
+    (show ascommodityspaced)
+    (show asprecision)
+    (show asdecimalpoint)
+    (show asdigitgroups)
+
 -- | A style for displaying digit groups in the integer part of a
 -- floating point number. It consists of the character used to
 -- separate groups (comma or period, whichever is not used as decimal