parsing: make balance assertions more robust with different precisions (#119)

Also note another issue.

hledger-lib/Hledger/Data/Amount.hs

@@ -81,6 +81,7 @@ module Hledger.Data.Amount (
   divideMixedAmount,
   isNegativeMixedAmount,
   isZeroMixedAmount,
+  isReallyZeroMixedAmount,
   isReallyZeroMixedAmountCost,
   -- ** rendering
   showMixedAmount,

hledger-lib/Hledger/Data/Journal.hs

@@ -391,7 +391,9 @@ checkBalanceAssertion :: ([String],MixedAmount) -> [Posting] -> ([String],MixedA
 checkBalanceAssertion (errs,bal) ps
   | null ps = (errs,bal)
   | isNothing assertion = (errs,bal)
-  | bal' /= assertedbal = (errs++[err], bal')
+  -- | bal' /= assertedbal  -- MixedAmount's Eq instance currently gets confused by different precisions
+  | not $ isReallyZeroMixedAmount (bal' - assertedbal)
+      = (errs++[err], bal')
   | otherwise = (errs,bal')
   where
     p = last ps

tests/balance-assertions.test

@@ -69,3 +69,45 @@ hledgerdev -f - stats
 >>> /Transactions/
 >>>2
 >>>=0
+
+# 5. should work for fractional amount with trailing zeros
+hledgerdev -f - stats
+<<<
+2013/1/1
+  a    $1.20  =$1.20
+  b           =-$1.20
+
+2013/1/2
+  a    $0.10  =$1.3
+  b           =-$1.3
+
+2013/1/3
+  a    $0.7   =$2
+  b           =-$2
+
+>>> /Transactions/
+>>>2
+>>>=0
+
+# 6. what should happen here ? Currently,
+# in a, 3.4 EUR @@ $5.6 and -3.4 EUR cancel out (wrong ?)
+# in b, 
+# 
+# hledgerdev -f - stats
+# <<<
+# 2013/1/1
+#   a    $1.20
+#   a    3.4 EUR @@ $5.6
+#   b  
+
+# 2013/1/2
+#   a    -3.4 EUR
+#   b
+
+# 2013/1/3
+#   a    $0.1   =$1.30
+#   b           =-$1.30
+
+# >>> /Transactions/
+# >>>2
+# >>>=0