Fix build-failure regression for base < 4.7

The `$>` operator definition is available only since base-4.7 which
unfortunately wasn't caught before release to Hackage (but has been
fixed up by a metadata-revision)

This commit introduces a `CompatPrelude` module which allows to reduce
the amount of CPP by ousting it to a central location, i.e. the new
`CompatPrelude` module. This pattern also tends to reduce the tricks
needed to silence unused import warnings.

Addresses #1119

1 files changed, 52 insertions, 0 deletions

diff --git a/haddock-library/src/CompatPrelude.hs b/haddock-library/src/CompatPrelude.hs
new file mode 100644
index 00000000..60fa94d9
--- /dev/null
+++ b/haddock-library/src/CompatPrelude.hs
@@ -0,0 +1,52 @@
+{-# LANGUAGE CPP #-}
+
+#if !MIN_VERSION_base(4,5,0)
+# error This module doesn't provide compat-shims for versions prior to base-4.5
+#endif
+
+-- | Bridge impedance mismatch of different @base@ versions back till @base-4.5@ (GHC 7.4.2)
+module CompatPrelude
+    ( ($>)
+    , isSymbolChar
+    ) where
+
+#if MIN_VERSION_base(4,7,0)
+import           Data.Functor                ( ($>) )
+#else
+import           Data.Functor                ( (<$) )
+#endif
+
+#if MIN_VERSION_base(4,9,0)
+import           Text.Read.Lex                      (isSymbolChar)
+#else
+import           Data.Char (GeneralCategory(..), generalCategory)
+#endif
+
+
+#if !MIN_VERSION_base(4,7,0)
+infixl 4 $>
+
+-- | Flipped version of '<$'.
+--
+-- @since 4.7.0.0
+($>) :: Functor f => f a -> b -> f b
+($>) = flip (<$)
+#endif
+
+#if !MIN_VERSION_base(4,9,0)
+-- inlined from base-4.10.0.0
+isSymbolChar :: Char -> Bool
+isSymbolChar c = not (isPuncChar c) && case generalCategory c of
+    MathSymbol           -> True
+    CurrencySymbol       -> True
+    ModifierSymbol       -> True
+    OtherSymbol          -> True
+    DashPunctuation      -> True
+    OtherPunctuation     -> c `notElem` "'\""
+    ConnectorPunctuation -> c /= '_'
+    _                    -> False
+  where
+    -- | The @special@ character class as defined in the Haskell Report.
+    isPuncChar :: Char -> Bool
+    isPuncChar = (`elem` (",;()[]{}`" :: String))
+#endif