Incomplete output when hPutStr is lifted

[andrew cooke]

Tomasz Zielonka said:
> On Thu, Nov 27, 2003 at 04:09:00PM -0300, andrew cooke wrote:
[...]
>> If I compile and run the code below, the file "foo" contains 10 lines
>> of output (as I would expect), but the file "bar" contains just 9 -
>> the final line is missing.  If I add a "join", as in the comment, then
>> all 10 lines appear.
>>
>> I don't understand why, completely - my best guess is that it's
>> related laziness and the final result of the fold not being used.
>
> No, it's not about laziness. The last result (action of type IO Int) is
> not threaded through the IO monad (forgive me the clumsy wording).

Ah.  Thanks.  I understand what you mean in general terms - I'll have to
go back and look at the code to understand in detail.

[...]
> Join will work, but why are you using such a strange code anyway ?

It was a simplified version of more complex code that drives a progress
meter - as the list is consumed IO is performed.  It wasn't designed from
the bottom up to support IO in such a low-level part of the code. The
simplest solution was to have the list be of "IO a" rather than "a" (the
IO coming from teh changing state of the progress meter).  Then the fold
over the list values gives code similar to what I posted.

Hope that makes some sense.  I'm still getting to grips with IO - it seems
a lot less complicated than I first thought, but I still don't have much
practice at structuring programs that use it.  So it's possible it's a
rather odd solution.

Cheers,
Andrew

-- 
personal web site: http://www.acooke.org/andrew
personal mail list: http://www.acooke.org/andrew/compute.html