[Haskell-cafe] Getting WriterT log lazily

[Magnus Therning]

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.



/M

--
Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
magnus＠therning．org          Jabber: magnus＠therning．org
http://therning.org/magnus         identi.ca|twitter: magthe

The goal is of course to use code along the same shape to do something more useful, and then `unsafePerformIO` will really be unsafe...

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