fix formatting of negative decimals (fixes #400)

src/Disco/Pretty.hs

@@ -146,17 +146,20 @@ prettyRational r
 --   the format @nnn.prefix[rep]...@, with any repeating digits enclosed
 --   in square brackets.
 prettyDecimal :: Rational -> String
-prettyDecimal r = printedDecimal
+prettyDecimal r = wholePart ++ "." ++ decimalPart
  where
-  (n, d) = properFraction r :: (Integer, Rational)
+  (n', d') = properFraction r :: (Integer, Rational)
+  d = abs d'
+  n = abs n'
   (expan, len) = digitalExpansion 10 (numerator d) (denominator d)
-  printedDecimal
+  wholePart = (if d' < 0 then "-" else "") ++ show n
+  decimalPart
     | length first102 > 101 || length first102 == 101 && last first102 /= 0 =
-        show n ++ "." ++ concatMap show (take 100 expan) ++ "..."
+        concatMap show (take 100 expan) ++ "..."
     | rep == [0] =
-        show n ++ "." ++ (if null pre then "0" else concatMap show pre)
+        if null pre then "0" else concatMap show pre
     | otherwise =
-        show n ++ "." ++ concatMap show pre ++ "[" ++ concatMap show rep ++ "]"
+        concatMap show pre ++ "[" ++ concatMap show rep ++ "]"
    where
     (pre, rep) = splitAt len expan
     first102 = take 102 expan

test/syntax-decimals/expected

@@ -22,3 +22,17 @@ T
 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000...
+-0.1
+-1.2
+-0.9
+0
+-0.1
+-0.15
+-0.0003
+-1.0003
+-1.1
+-0.999
+-1.3
+-104.55
+-104.0
+-1.0

test/syntax-decimals/input

@@ -22,3 +22,17 @@
 (1.0/10^99)
 (1.0/10^100)
 (1.0/10^101)
+0.1 - 0.2
+0.2 * -6
+0.3 * (-0.3)
+-0.0
+0.0 - 0.1
+0.0 - 0.15
+0.0 - 0.0003
+0.0 - 1.0003
+0.0 - 1.1
+0.0 - 0.999
+0.0 - 1.3
+0.0 - 104.55
+0.0 - 104.0
+0.0 - 1.0