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

6 files changed, 61 insertions, 30 deletions

diff --git a/haddock-library/CHANGES.md b/haddock-library/CHANGES.md
index 265579ca..d112db45 100644
--- a/haddock-library/CHANGES.md
+++ b/haddock-library/CHANGES.md
@@ -1,3 +1,7 @@
+## Changes in version 1.8.0.1
+
+ * Fix build-time regression for `base < 4.7` (#1119)
+
 ## Changes in version 1.8.0
 
  * Support inline markup in markdown-style links (#875)
diff --git a/haddock-library/haddock-library.cabal b/haddock-library/haddock-library.cabal
index fe6aeede..7f91fd19 100644
--- a/haddock-library/haddock-library.cabal
+++ b/haddock-library/haddock-library.cabal
@@ -1,6 +1,6 @@
 cabal-version:        2.2
 name:                 haddock-library
-version:              1.8.0
+version:              1.8.0.1
 synopsis:             Library exposing some functionality of Haddock.
 
 description:          Haddock is a documentation-generation tool for Haskell
@@ -49,6 +49,7 @@ library
     Documentation.Haddock.Types
 
   other-modules:
+    CompatPrelude
     Documentation.Haddock.Parser.Util
     Documentation.Haddock.Parser.Monad
     Documentation.Haddock.Parser.Identifier
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
diff --git a/haddock-library/src/Documentation/Haddock/Parser.hs b/haddock-library/src/Documentation/Haddock/Parser.hs
index 36c8bb5b..028422a7 100644
--- a/haddock-library/src/Documentation/Haddock/Parser.hs
+++ b/haddock-library/src/Documentation/Haddock/Parser.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE CPP               #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ViewPatterns      #-}
 -- |
diff --git a/haddock-library/src/Documentation/Haddock/Parser/Identifier.hs b/haddock-library/src/Documentation/Haddock/Parser/Identifier.hs
index a83e5abf..b8afb951 100644
--- a/haddock-library/src/Documentation/Haddock/Parser/Identifier.hs
+++ b/haddock-library/src/Documentation/Haddock/Parser/Identifier.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE CPP          #-}
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE ViewPatterns #-}
 -- |
@@ -29,15 +28,8 @@ import qualified Data.Text as T
 
 import           Data.Char (isAlpha, isAlphaNum)
 import Control.Monad (guard)
-import Data.Functor (($>))
-#if MIN_VERSION_base(4,9,0)
-import           Text.Read.Lex                      (isSymbolChar)
-#else
-import           Data.Char                          (GeneralCategory (..),
-                                                     generalCategory)
-#endif
-
 import Data.Maybe
+import CompatPrelude
 
 -- | Identifier string surrounded with namespace, opening, and closing quotes/backticks.
 data Identifier = Identifier !Namespace !Char String !Char
@@ -57,24 +49,6 @@ parseValid = do
       in setParserState s' $> Identifier ns op (T.unpack ident) cl
 
 
-#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
-
 -- | Try to parse a delimited identifier off the front of the given input.
 --
 -- This tries to match as many valid Haskell identifiers/operators as possible,
diff --git a/haddock-library/src/Documentation/Haddock/Parser/Monad.hs b/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
index fa46f536..7c73a168 100644
--- a/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
+++ b/haddock-library/src/Documentation/Haddock/Parser/Monad.hs
@@ -29,7 +29,6 @@ import qualified Data.Text as T
 import           Data.Text                   ( Text )
 
 import           Control.Monad               ( mfilter )
-import           Data.Functor                ( ($>) )
 import           Data.String                 ( IsString(..) )
 import           Data.Bits                   ( Bits(..) )
 import           Data.Char                   ( ord )
@@ -37,7 +36,9 @@ import           Data.List                   ( foldl' )
 import           Control.Applicative as App
 
 import           Documentation.Haddock.Types ( Version )
+
 import           Prelude hiding (takeWhile)
+import           CompatPrelude
 
 -- | The only bit of information we really care about truding along with us
 -- through parsing is the version attached to a @\@since@ annotation - if