Move `readMaybe` to Client.Prelude and remove dupes

cabal-install/Distribution/Client/Compat/Prelude.hs

+{-# LANGUAGE CPP #-}
+
 -- | This module does two things:
 --
 -- * Acts as a compatiblity layer, like @base-compat@.
@@ -10,7 +12,21 @@
 module Distribution.Client.Compat.Prelude
   ( module Distribution.Compat.Prelude
   , Prelude.IO
+  , readMaybe
   ) where
 
 import Prelude (IO)
 import Distribution.Compat.Prelude hiding (IO)
+
+#if MIN_VERSION_base(4,6,0)
+import Text.Read
+         ( readMaybe )
+#endif
+
+#if !MIN_VERSION_base(4,6,0)
+-- | An implementation of readMaybe, for compatability with older base versions.
+readMaybe :: Read a => String -> Maybe a
+readMaybe s = case reads s of
+                [(x,"")] -> Just x
+                _        -> Nothing
+#endif

cabal-install/Distribution/Client/HttpUtils.hs

@@ -41,7 +41,7 @@ import Distribution.Simple.Utils
          , copyFileVerbose,  withTempFile
          , rawSystemStdInOut, toUTF8, fromUTF8, normaliseLineEndings )
 import Distribution.Client.Utils
-         ( readMaybe, withTempFileName )
+         ( withTempFileName )
 import Distribution.Client.Types
          ( RemoteRepo(..) )
 import Distribution.System

cabal-install/Distribution/Client/Init.hs

@@ -603,11 +603,6 @@ promptList' displayItem numChoices choices def other = do
                                   = return . Right $ choices !! (n-1)
                    | otherwise    = Left `fmap` promptStr "Please specify" Nothing
 
-readMaybe :: (Read a) => String -> Maybe a
-readMaybe s = case reads s of
-                [(a,"")] -> Just a
-                _        -> Nothing
-
 ---------------------------------------------------------------------------
 --  File generation  ------------------------------------------------------
 ---------------------------------------------------------------------------

cabal-install/Distribution/Client/Setup.hs

@@ -1772,9 +1772,6 @@ initCommand = CommandUI {
       , optionVerbosity IT.initVerbosity (\v flags -> flags { IT.initVerbosity = v })
       ]
   }
-  where readMaybe s = case reads s of
-                        [(x,"")]  -> Just x
-                        _         -> Nothing
 
 -- ------------------------------------------------------------
 -- * SDist flags

cabal-install/Distribution/Client/Utils.hs

@@ -31,10 +31,6 @@ import qualified Data.ByteString.Lazy as BS
 import Data.Bits
          ( (.|.), shiftL, shiftR )
 import System.FilePath
-#if MIN_VERSION_base(4,6,0)
-import Text.Read
-         ( readMaybe )
-#endif
 import Data.List
          ( groupBy )
 import Data.Word
@@ -93,14 +89,6 @@ duplicatesBy cmp = filter moreThanOne . groupBy eq . sortBy cmp
     moreThanOne (_:_:_) = True
     moreThanOne _       = False
 
-#if !MIN_VERSION_base(4,6,0)
--- | An implementation of readMaybe, for compatability with older base versions.
-readMaybe :: Read a => String -> Maybe a
-readMaybe s = case reads s of
-                [(x,"")] -> Just x
-                _        -> Nothing
-#endif
-
 -- | Like 'removeFile', but does not throw an exception when the file does not
 -- exist.
 removeExistingFile :: FilePath -> IO ()