[Haskell-cafe] Getting WriterT log lazily

[Ryan Ingram]

How about this:

> type ActionLog v = Writer [IO v]

> myTell :: v -> ActionLog v ()
> myTell a = tell [sleep 1 >> return a]

> foo :: ActionLog Int ()
> foo = mapM_ myTell [1..10]

> main = sequence_ results where
>    (_, vals) = runWriter foo
>    results = map (>>= print) vals

  -- ryan

On Sun, May 3, 2009 at 2:17 PM, Magnus Therning <magnus at therning.org> wrote:
> I've been playing around with (WriterT [Int] IO), trying to get the log
> out and map `print` over it... and do it lazily.  However, I'm not
> really happy with what I have so far, since I've had to resort to
> `unsafePerformIO`.  Any hints are welcome.
>
> What I have so far is:
>
>  foo = let
>          _tell i = do
>              a <- return $ unsafePerformIO $ sleep 1
>              tell [a + 1 `seq` i]
>      in do
>          mapM_ _tell [1..10]
>
>  main = do
>      (_, ~res) <- runWriterT foo
>      mapM_ print res
>
> Without the `seq` the call to sleep will simply be skipped (is there an
> easier way to force evaluation there?).  Without `unsafePerformIO` all
> the sleeping is done up front, and all numbers are print at once at the
> end.
>
> The goal is of course to use code along the same shape to do something more
> useful, and then `unsafePerformIO` will really be unsafe...
>
> /M
>
> --
> Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
> magnus＠therning．org          Jabber: magnus＠therning．org
> http://therning.org/magnus         identi.ca|twitter: magthe
>
>
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