[project @ 1997-12-04 14:57:34 by simonm]

[ghc-hetmet.git] / ghc / lib / ghc / UnsafeST.lhs

diff --git a/ghc/lib/ghc/UnsafeST.lhs b/ghc/lib/ghc/UnsafeST.lhs
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+%
+% (c) The AQUA Project, Glasgow University, 1994-1996
+%
+
+\section[UnsafeST]{Module @UnsafeST@}
+
+These functions have their own module because we definitely don't want
+them to be inlined.
+
+\begin{code}
+{-# OPTIONS -fno-implicit-prelude #-}
+
+module UnsafeST (unsafeInterleaveST, runST) where
+
+import STBase
+import PrelBase
+\end{code}
+
+\begin{code}
+unsafeInterleaveST :: ST s a -> ST s a
+unsafeInterleaveST (ST m) = ST ( \ s ->
+    let
+       STret _ r = m s
+    in
+    STret s r)
+
+\end{code}
+
+Definition of runST
+~~~~~~~~~~~~~~~~~~~
+
+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'' )
+\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'
+      in
+      freezeArray# a s''
+\end{verbatim}
+And now the @newArray#@ binding can be floated to become a CAF, which
+is totally and utterly wrong:
+\begin{verbatim}
+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''
+\end{verbatim}
+All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.
+
+\begin{code}
+runST :: (All s => ST s a) -> a
+runST st = 
+  case st of
+       ST m -> case m realWorld# of
+                       STret _ r -> r
+\end{code}
+