From: ross <unknown>
Date: Thu, 13 Oct 2005 10:35:49 +0000 (+0000)
Subject: [project @ 2005-10-13 10:35:49 by ross]
X-Git-Tag: cmm-merge2~14
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=1364d50a755a360a9d567ff3ac229649af16ac47;p=ghc-base.git

[project @ 2005-10-13 10:35:49 by ross]
comments re strict/lazy monads
---

diff --git a/Control/Monad/ST/Lazy.hs b/Control/Monad/ST/Lazy.hs
index a160cdf..5bf1265 100644
--- a/Control/Monad/ST/Lazy.hs
+++ b/Control/Monad/ST/Lazy.hs
@@ -9,8 +9,8 @@
 -- 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.
 --
 -----------------------------------------------------------------------------
 
@@ -61,6 +61,10 @@ import Hugs.LazyST
 --
 -- 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)
 
diff --git a/GHC/ST.lhs b/GHC/ST.lhs
index 4bb4256..909a8da 100644
--- a/GHC/ST.lhs
+++ b/GHC/ST.lhs
@@ -46,6 +46,11 @@ too many people got bitten by space leaks when it was lazy.
 -- 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 #)