Invert module-dep between Control.Monad and Data.Foldable

This is the last preparation needed before generalizing entities in
Control.Monad conflicting with those from Data.Foldable (re #9586)

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D225

5 files changed, 16 insertions, 6 deletions

diff --git a/libraries/base/Control/Applicative.hs b/libraries/base/Control/Applicative.hs
index accf58f561..d6157b3d69 100644
--- a/libraries/base/Control/Applicative.hs
+++ b/libraries/base/Control/Applicative.hs
@@ -53,6 +53,7 @@ import Data.Maybe
 import Data.Tuple
 import Data.Eq
 import Data.Ord
+import Data.Foldable (Foldable(..))
 import Data.Functor ((<$>))
 
 import GHC.Base hiding ((.), id)
@@ -64,6 +65,9 @@ import GHC.Show (Show)
 newtype Const a b = Const { getConst :: a }
                   deriving (Generic, Generic1)
 
+instance Foldable (Const m) where
+    foldMap _ _ = mempty
+
 instance Functor (Const m) where
     fmap _ (Const v) = Const v
 
diff --git a/libraries/base/Control/Monad.hs b/libraries/base/Control/Monad.hs
index 0597055da9..3487a09c65 100644
--- a/libraries/base/Control/Monad.hs
+++ b/libraries/base/Control/Monad.hs
@@ -76,6 +76,7 @@ module Control.Monad
     , (<$!>)
     ) where
 
+import Data.Foldable ()
 import Data.Functor ( void )
 import Data.Maybe
 
diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index 0e655de573..f6f787b628 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -58,7 +58,6 @@ module Data.Foldable (
     find
     ) where
 
-import Control.Applicative ( Const )
 import Data.Bool
 import Data.Either
 import Data.Eq
@@ -202,9 +201,6 @@ instance Foldable Proxy where
     foldr1 _ _ = error "foldr1: Proxy"
     {-# INLINE foldr1 #-}
 
-instance Foldable (Const m) where
-    foldMap _ _ = mempty
-
 -- | Monadic fold over the elements of a structure,
 -- associating to the right, i.e. from right to left.
 foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m b
diff --git a/libraries/base/Text/ParserCombinators/ReadP.hs b/libraries/base/Text/ParserCombinators/ReadP.hs
index 3d2b39c57e..0139e7733d 100644
--- a/libraries/base/Text/ParserCombinators/ReadP.hs
+++ b/libraries/base/Text/ParserCombinators/ReadP.hs
@@ -72,7 +72,6 @@ module Text.ParserCombinators.ReadP
   )
  where
 
-import Control.Monad ( sequence )
 import {-# SOURCE #-} GHC.Unicode ( isSpace )
 import GHC.List ( replicate, null )
 import GHC.Base hiding ( many )
@@ -311,6 +310,10 @@ count :: Int -> ReadP a -> ReadP [a]
 -- ^ @count n p@ parses @n@ occurrences of @p@ in sequence. A list of
 --   results is returned.
 count n p = sequence (replicate n p)
+  where -- local 'sequence' to avoid import-cycle
+    sequence ms = foldr k (return []) ms
+      where
+        k m m' = do { x <- m; xs <- m'; return (x:xs) }
 
 between :: ReadP open -> ReadP close -> ReadP a -> ReadP a
 -- ^ @between open close p@ parses @open@, followed by @p@ and finally
diff --git a/libraries/base/Text/Read/Lex.hs b/libraries/base/Text/Read/Lex.hs
index 557637d896..39ca46a33f 100644
--- a/libraries/base/Text/Read/Lex.hs
+++ b/libraries/base/Text/Read/Lex.hs
@@ -45,7 +45,13 @@ import GHC.Real( Rational, (%), fromIntegral,
 import GHC.List
 import GHC.Enum( minBound, maxBound )
 import Data.Maybe
-import Control.Monad
+
+-- local copy to break import-cycle
+-- | @'guard' b@ is @'return' ()@ if @b@ is 'True',
+-- and 'mzero' if @b@ is 'False'.
+guard           :: (MonadPlus m) => Bool -> m ()
+guard True      =  return ()
+guard False     =  mzero
 
 -- -----------------------------------------------------------------------------
 -- Lexing types