Correct doctests

It's simpler to assume that base is NoImplicitPrelude,
otherwise running doctest on `GHC.*` modules would be tricky.
OTOH, most `GHC.List` (where the most name clashes are) examples
could be changed to use `import qualified Data.List as L`.
(GHC.List examples won't show for Foldable methods...).

With these changes majority of doctest examples are GHCi-"faithful",
my WIP GHC-independent doctest runner reports nice summary:

Examples: 582; Tried: 546; Skipped: 34; Success: 515; Errors: 33; Property Failures 2

Most error cases are *Hangs forever*.
I have yet to figure out how to demonstrate that in GHCi.
Some of divergences are actually stack overflows, i.e. caught by
runtime.

Few errorful cases are examples of infinite output, e.g.

    >>> cycle [42]
    [42,42,42,42,42,42,42,42,42,42...

while correct, they confuse doctest.

Another erroneous cases are where expected output has line comment, like

    >>> fmap show (Just 1)  --  (a   -> b)      -> f a       -> f b
    Just "1"                --  (Int -> String) -> Maybe Int -> Maybe String

I think I just have to teach doctest to strip comments from expected
output.

This is a first patch in a series.
There is plenty of stuff already.

1 files changed, 3 insertions, 0 deletions

diff --git a/libraries/base/Data/Unique.hs b/libraries/base/Data/Unique.hs
index 95b94102d1..c14b063b3b 100644
--- a/libraries/base/Data/Unique.hs
+++ b/libraries/base/Data/Unique.hs
@@ -26,6 +26,9 @@ import System.IO.Unsafe (unsafePerformIO)
 import GHC.Num
 import Data.IORef
 
+-- $setup
+-- >>> import Prelude
+
 -- | An abstract unique object.  Objects of type 'Unique' may be
 -- compared for equality and ordering and hashed into 'Int'.
 --