Hi Emil,
The reason has to do with the definitions of (>>=) for Writer and
(WriterT m). Looking at Control.Monad.Writer (ghc-6.2.2),
newtype Writer w a = Writer { runWriter :: (a, w) }
instance (Monoid w) => Monad (Writer w) where
m >>= k = Writer $ let
(a, w) = runWriter m
(b, w') = runWriter (k a)
in (b, w `mappend` w')
newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }
instance (Monoid w, Monad m) => Monad (WriterT w m) where
return a = WriterT $ return (a, mempty)
m >>= k = WriterT $ do
(a, w) <- runWriterT m
(b, w') <- runWriterT (k a)
return (b, w `mappend` w')
Patterns in "let" expressions bind lazily, so Writer's (>>=) is lazy
in both its arguments and thus can handle the infinite recursion of
your "foo". However, patterns in "do" expressions bind strictly, so
WriterT's (>>=) is strict in its arguments; it tries to evalue "foo"
completely, causing a stack overflow.
You may use "case" expressions instead of "let" statements to bind
patterns strictly. Conversely, you can also make a do statement bind
patterns lazily, using lazy patterns (see eg
http://www.cs.sfu.ca/CC/SW/Haskell/hugs/tutorial-1.4-html/patterns.html#tut-lazy-patterns)
Hope that helps,
-Judah
On Tue, 15 Feb 2005 17:45:26 +0100, Emil Axelsson <[EMAIL PROTECTED]> wrote:
> Hello,
>
> I have a huge space leak in a program due to laziness in the writer monad. Now
> when I'm trying to examine the behaviour of writer I get a bit puzzled by the
> following program:
>
> foo :: MonadWriter [Int] m => Int -> m ()
> foo n = do tell [n]
> foo $ n+1
>
> test = (snd $ runWriter $ foo 0) !! 3
>
> testT = (snd $ runIdentity $ runWriterT $ foo 0) !! 3
>
> I would expect both test and testT to terminate with the value 3, due to the
> laziness that caused me problems. But here is the actual run results:
>
> Ok, modules loaded: Main.
> *Main> test
> 3
> *Main> testT
> *** Exception: stack overflow
> *Main>
>
> Could someone please explain this to me?
>
> / Emil
>
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