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

[ghc-base.git] / GHC / ST.lhs

diff --git a/GHC/ST.lhs b/GHC/ST.lhs
index ab1a1b1..dc62000 100644 (file)
--- a/GHC/ST.lhs
+++ b/GHC/ST.lhs
@@ -1,5 +1,6 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples, Rank2Types #-}
+{-# OPTIONS_HADDOCK hide #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.ST
@@ -14,25 +15,42 @@
 --
 -----------------------------------------------------------------------------
 
+-- #hide
 module GHC.ST where
 
 import GHC.Base
 import GHC.Show
-import GHC.Num
 
 default ()
 \end{code}
 
 %*********************************************************
-%*                                                     *
+%*                                                      *
 \subsection{The @ST@ monad}
-%*                                                     *
+%*                                                      *
 %*********************************************************
 
 The state-transformer monad proper.  By default the monad is strict;
 too many people got bitten by space leaks when it was lazy.
 
 \begin{code}
+-- | The strict state-transformer monad.
+-- 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 uninstantiated type variable (inside invocations of 'runST'), or
+--
+-- * 'RealWorld' (inside invocations of 'Control.Monad.ST.stToIO').
+--
+-- It serves to keep the internal states of different invocations
+-- of 'runST' separate from each other and from invocations of
+-- 'Control.Monad.ST.stToIO'.
+--
+-- The '>>=' and '>>' operations are strict in the state (though not in
+-- values stored in the state).  For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= f) = _|_@
 newtype ST s a = ST (STRep s a)
 type STRep s a = State# s -> (# State# s, a #)
 
@@ -45,14 +63,14 @@ instance Monad (ST s) where
     {-# INLINE return #-}
     {-# INLINE (>>)   #-}
     {-# INLINE (>>=)  #-}
-    return x = ST $ \ s -> (# s, x #)
-    m >> k   =  m >>= \ _ -> k
+    return x = ST (\ s -> (# s, x #))
+    m >> k   = m >>= \ _ -> k
 
     (ST m) >>= k
-      = ST $ \ s ->
-       case (m s) of { (# new_s, r #) ->
-       case (k r) of { ST k2 ->
-       (k2 new_s) }}
+      = ST (\ s ->
+        case (m s) of { (# new_s, r #) ->
+        case (k r) of { ST k2 ->
+        (k2 new_s) }})
 
 data STret s a = STret (State# s) a
 
@@ -65,21 +83,24 @@ liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r
 unsafeInterleaveST :: ST s a -> ST s a
 unsafeInterleaveST (ST m) = ST ( \ s ->
     let
-       r = case m s of (# _, res #) -> res
+        r = case m s of (# _, res #) -> res
     in
     (# s, r #)
   )
 
+-- | Allow the result of a state transformer computation to be used (lazily)
+-- inside the computation.
+-- Note that if @f@ is strict, @'fixST' f = _|_@.
 fixST :: (a -> ST s a) -> ST s a
 fixST k = ST $ \ s ->
     let ans       = liftST (k r) s
-       STret _ r = ans
+        STret _ r = ans
     in
     case ans of STret s' x -> (# s', x #)
 
 instance  Show (ST s a)  where
     showsPrec _ _  = showString "<<ST action>>"
-    showList      = showList__ (showsPrec 0)
+    showList       = showList__ (showsPrec 0)
 \end{code}
 
 Definition of runST
@@ -89,16 +110,16 @@ SLPJ 95/04: Why @runST@ must not have an unfolding; consider:
 \begin{verbatim}
 f x =
   runST ( \ s -> let
-                   (a, s')  = newArray# 100 [] s
-                   (_, s'') = fill_in_array_or_something a x s'
-                 in
-                 freezeArray# a s'' )
+                    (a, s')  = newArray# 100 [] s
+                    (_, s'') = fill_in_array_or_something a x s'
+                  in
+                  freezeArray# a s'' )
 \end{verbatim}
 If we inline @runST@, we'll get:
 \begin{verbatim}
 f x = let
-       (a, s')  = newArray# 100 [] realWorld#{-NB-}
-       (_, s'') = fill_in_array_or_something a x s'
+        (a, s')  = newArray# 100 [] realWorld#{-NB-}
+        (_, s'') = fill_in_array_or_something a x s'
       in
       freezeArray# a s''
 \end{verbatim}
@@ -109,8 +130,8 @@ f = let
     (a, s')  = newArray# 100 [] realWorld#{-NB-} -- YIKES!!!
     in
     \ x ->
-       let (_, s'') = fill_in_array_or_something a x s' in
-       freezeArray# a s''
+        let (_, s'') = fill_in_array_or_something a x s' in
+        freezeArray# a s''
 \end{verbatim}
 All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.
 
@@ -119,14 +140,25 @@ All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.
 -- The INLINE prevents runSTRep getting inlined in *this* module
 -- so that it is still visible when runST is inlined in an importing
 -- module.  Regrettably delicate.  runST is behaving like a wrapper.
+
+-- | Return the value computed by a state transformer computation.
+-- 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 = runSTRep (case st of { ST st_rep -> st_rep })
 
 -- I'm only letting runSTRep be inlined right at the end, in particular *after* full laziness
 -- That's what the "INLINE [0]" says.
---             SLPJ Apr 99
-{-# INLINE [0] runSTRep #-}
+--              SLPJ Apr 99
+-- {-# INLINE [0] runSTRep #-}
+
+-- SDM: further to the above, inline phase 0 is run *before*
+-- full-laziness at the moment, which means that the above comment is
+-- invalid.  Inlining runSTRep doesn't make a huge amount of
+-- difference, anyway.  Hence:
+
+{-# NOINLINE runSTRep #-}
 runSTRep :: (forall s. STRep s a) -> a
 runSTRep st_rep = case st_rep realWorld# of
-                       (# _, r #) -> r
+                        (# _, r #) -> r
 \end{code}