Ketil Malde wrote:
> I've done something similar, I think. Often, I want to output some
> kind of progress indicator, just to show that the program is working.
> Typically, the program works by lazily evaluating a list (lines from
> an input file, say); each element of the list is wrapped with an IO
> action that outputs the status when evaluated -- which typically
> happens lazily from pure code.
>
>> countIO :: String -> String -> Int -> [a] -> IO [a]
>> countIO msg post step xs = sequence $ map unsafeInterleaveIO ((blank >>
>> outmsg (0::Int) >> c):cs)
>> where (c:cs) = ct 0 xs
>> output = hPutStr stderr
>> blank = output ('\r':take 70 (repeat ' '))
>> outmsg x = output ('\r':msg++show x) >> hFlush stderr
>> ct s ys = let (a,b) = splitAt (step-1) ys
>> next = s+step
>> in case b of [b1] -> map return a ++ [outmsg (s+step) >>
>> hPutStr stderr post >> return b1]
>> [] -> map return (init a) ++ [outmsg
>> (s+length a) >> hPutStr stderr post >> return (last a)]
>> _ -> map return a ++ [outmsg s >> return
>> (head b)] ++ ct next (tail b)
>
> -k
Your use of unsafeInterleaveIO is just not quite correct. A quick series of
examples:
Let me define this function:
> unsafeSequenceIO :: [ IO a ] -> IO [a]
> unsafeSequenceIO [] = return []
> unsafeSequenceIO (x:xs) = unsafeInterleaveIO $ do
> this <- x
> rest <- unsafeSequenceIO xs
> return (this:rest)
And an infinite [ IO a ]
> todo :: [ IO Int ]
> todo = map return [0..]
These diverge
> *Main> liftM (take 10) (sequence todo)
> *Main> liftM (take 10) (sequence (map unsafeInterleaveIO todo))
This is finite:
> *Main> liftM (take 10) (unsafeSequenceIO todo)
> [0,1,2,3,4,5,6,7,8,9]
An alternate definition of unsafeSequenceIO, which is not quite the same but
still works, is
> unsafeSequenceIO' :: [ IO a ] -> IO [a]
> unsafeSequenceIO' [] = return []
> unsafeSequenceIO' (x:xs) = do
> this <- x
> rest <- unsafeInterleaveIO' (unsafeSequenceIO' xs)
> return (this:rest)
The two definitions differ only very slightly in how lazily the very first
element is handled:
> *Main> (unsafeSequenceIO (error "boom":[]) >> print "ok")
> "ok"
> *Main> (unsafeSequenceIO' (error "boom":[]) >> print "ok")
> *** Exception: boom
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