Use explicit language extensions & remove extension fields from base.cabal

[ghc-base.git] / Control / Monad / ST / Lazy.hs

diff --git a/Control/Monad/ST/Lazy.hs b/Control/Monad/ST/Lazy.hs
index b91e062..8aac665 100644 (file)
--- a/Control/Monad/ST/Lazy.hs
+++ b/Control/Monad/ST/Lazy.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE CPP, MagicHash, UnboxedTuples, Rank2Types #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Monad.ST.Lazy
@@ -9,40 +11,38 @@
 -- Portability :  non-portable (requires universal quantification for runST)
 --
 -- This module presents an identical interface to "Control.Monad.ST",
--- but the underlying implementation of the state thread is /lazy/ (in
--- the sense that (@_|_ >> a@ is not necessarily equal to @_|_@).
+-- except that the monad delays evaluation of state operations until
+-- a value depending on them is required.
 --
 -----------------------------------------------------------------------------
 
 module Control.Monad.ST.Lazy (
-       -- * The 'ST' monad
-       ST,
-       runST,
-       fixST,
+        -- * The 'ST' monad
+        ST,
+        runST,
+        fixST,
 
-       -- * Converting between strict and lazy 'ST'
-       strictToLazyST, lazyToStrictST,
+        -- * Converting between strict and lazy 'ST'
+        strictToLazyST, lazyToStrictST,
 
-       -- * Converting 'ST' To 'IO'
-       RealWorld,
-       stToIO,
+        -- * Converting 'ST' To 'IO'
+        RealWorld,
+        stToIO,
 
-       -- * Unsafe operations
-       unsafeInterleaveST,
-       unsafeIOToST
+        -- * Unsafe operations
+        unsafeInterleaveST,
+        unsafeIOToST
     ) where
 
 import Prelude
 
 import Control.Monad.Fix
 
-import Control.Monad.ST (RealWorld)
 import qualified Control.Monad.ST as ST
 
 #ifdef __GLASGOW_HASKELL__
 import qualified GHC.ST
 import GHC.Base
-import Control.Monad
 #endif
 
 #ifdef __HUGS__
@@ -51,10 +51,20 @@ import Hugs.LazyST
 
 #ifdef __GLASGOW_HASKELL__
 -- | The lazy state-transformer monad.
--- The first parameter is used solely to keep the states of different
--- invocations of 'runST' separate from each other and from invocations
--- of 'Control.Monad.ST.stToIO'.  In the first case the type parameter
--- is not instantiated; in the second it is 'RealWorld'.
+-- A computation of type @'ST' s a@ transforms an internal state indexed
+-- by @s@, and returns a value of type @a@.
+-- The @s@ parameter is either
+--
+-- * an unstantiated type variable (inside invocations of 'runST'), or
+--
+-- * 'RealWorld' (inside invocations of 'stToIO').
+--
+-- It serves to keep the internal states of different invocations of
+-- 'runST' separate from each other and from invocations of 'stToIO'.
+--
+-- The '>>=' and '>>' operations are not strict in the state.  For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2@
 newtype ST s a = ST (State s -> (a, State s))
 data State s = S# (State# s)
 
@@ -70,7 +80,7 @@ instance Monad (ST s) where
 
         return a = ST $ \ s -> (a,s)
         m >> k   =  m >>= \ _ -> k
-       fail s   = error s
+        fail s   = error s
 
         (ST m) >>= k
          = ST $ \ s ->
@@ -82,25 +92,25 @@ instance Monad (ST s) where
 
 {-# NOINLINE runST #-}
 -- | Return the value computed by a state transformer computation.
--- The @forall@ is a technical device to ensure that the state used
--- by the 'ST' computation is inaccessible to the rest of the program.
+-- The @forall@ ensures that the internal state used by the 'ST'
+-- computation is inaccessible to the rest of the program.
 runST :: (forall s. ST s a) -> a
 runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
 
 -- | Allow the result of a state transformer computation to be used (lazily)
 -- inside the computation.
--- Note that if @f@ is strict, @'fixST' f@ will diverge.
+-- Note that if @f@ is strict, @'fixST' f = _|_@.
 fixST :: (a -> ST s a) -> ST s a
 fixST m = ST (\ s -> 
-               let 
-                  ST m_r = m r
-                  (r,s') = m_r s
-               in
-                  (r,s'))
+                let 
+                   ST m_r = m r
+                   (r,s') = m_r s
+                in
+                   (r,s'))
 #endif
 
 instance MonadFix (ST s) where
-       mfix = fixST
+        mfix = fixST
 
 -- ---------------------------------------------------------------------------
 -- Strict <--> Lazy
@@ -114,11 +124,11 @@ the lazy state thread it returns is demanded.
 strictToLazyST :: ST.ST s a -> ST s a
 strictToLazyST m = ST $ \s ->
         let 
-          pr = case s of { S# s# -> GHC.ST.liftST m s# }
-          r  = case pr of { GHC.ST.STret _ v -> v }
-          s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
-       in
-       (r, s')
+           pr = case s of { S# s# -> GHC.ST.liftST m s# }
+           r  = case pr of { GHC.ST.STret _ v -> v }
+           s' = case pr of { GHC.ST.STret s2# _ -> S# s2# }
+        in
+        (r, s')
 
 {-| 
 Convert a lazy 'ST' computation into a strict one.
@@ -135,7 +145,8 @@ unsafeIOToST :: IO a -> ST s a
 unsafeIOToST = strictToLazyST . ST.unsafeIOToST
 
 -- | A monad transformer embedding lazy state transformers in the 'IO'
--- monad.  The 'RealWorld' parameter is a technical device to keep the
--- state used by such computations separate from those inside 'runST'.
+-- monad.  The 'RealWorld' parameter indicates that the internal state
+-- used by the 'ST' computation is a special one supplied by the 'IO'
+-- monad, and thus distinct from those used by invocations of 'runST'.
 stToIO :: ST RealWorld a -> IO a
 stToIO = ST.stToIO . lazyToStrictST