Surrounds negative numbers with parentheses when necessary

This behavior is consistent with Show instances of integral types in
base and allows the result of `show` to be parsed as Haskell code.
The added test used to fail and now passes:

```
    show (Just (-1)):                   FAIL
      test/test.hs:116:
      expected: "Just (-1.0)"
       but got: "Just -1.0"
```

changelog

@@ -21,6 +21,9 @@
 
 	* Make / on Scientifics slightly more efficient.
 
+	* Fix the Show instance to surround negative numbers with parentheses when
+	necessary.
+
 0.3.6.2
 	* Due to a regression introduced in 0.3.4.14 the RealFrac methods
 	and floatingOrInteger became vulnerable to a space blowup when

src/Data/Scientific/Internal.hs

@@ -953,11 +953,16 @@ isE c = c == 'e' || c == 'E'
 
 -- | See 'formatScientific' if you need more control over the rendering.
 instance Show Scientific where
-    show s | coefficient s < 0 = '-':showPositive (-s)
-           | otherwise         =     showPositive   s
+    showsPrec d s
+        | coefficient s < 0 = showParen (d > prefixMinusPrec) $
+               showChar '-' . showPositive (-s)
+        | otherwise         = showPositive   s
       where
-        showPositive :: Scientific -> String
-        showPositive = fmtAsGeneric . toDecimalDigits
+        prefixMinusPrec :: Int
+        prefixMinusPrec = 6
+
+        showPositive :: Scientific -> ShowS
+        showPositive = showString . fmtAsGeneric . toDecimalDigits
 
         fmtAsGeneric :: ([Int], Int) -> String
         fmtAsGeneric x@(_is, e)

test/test.hs

@@ -112,6 +112,9 @@ main = testMain $ testGroup "scientific"
 
   , testGroup "Formatting"
     [ testProperty "read . show == id" $ \s -> read (show s) === s
+    , testCase "show (Just 1)"    $ testShow (Just 1)    "Just 1.0"
+    , testCase "show (Just 0)"    $ testShow (Just 0)    "Just 0.0"
+    , testCase "show (Just (-1))" $ testShow (Just (-1)) "Just (-1.0)"
 
     , testGroup "toDecimalDigits"
       [ smallQuick "laws"
@@ -283,6 +286,9 @@ testReads inp out = reads inp @?= out
 testRead :: String -> Scientific -> Assertion
 testRead inp out = read inp @?= out
 
+testShow :: Maybe Scientific -> String -> Assertion
+testShow inp out = show inp @?= out
+
 testScientificP :: String -> [(Scientific, String)] -> Assertion
 testScientificP inp out = readP_to_S Scientific.scientificP inp @?= out