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state monad.  Check out the "safe-lazy-io".  I've never used it, but the
announcement is pretty convincing.
http://www.haskell.org/pipermail/haskell/2009-March/021133.html

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<br><br><div class=3D"gmail_quote">On Thu, Feb 24, 2011 at 2:42 PM, Andrew =
Coppin <span dir=3D"ltr">&lt;<a href=3D"mailto:andrewcoppin at btinternet.com"=
>andrewcoppin at btinternet.com</a>&gt;</span> wrote:<br><blockquote class=3D"=
gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-=
left:1ex;">
<div class=3D"im"><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .=
8ex;border-left:1px #ccc solid;padding-left:1ex">
 =A0 =A0Anybody have any hints on how to get around this?<br>
<br>
Use a lazy state monad?<br>
</blockquote>
<br></div>
That&#39;s not going to work. It still needs to read the input to determine=
 which monadic action comes next, and hence what the final result will be. =
So whether it forces the result or not, it still has to scan the entire inp=
ut before it can generate any output.<div>
<div></div><div class=3D"h5"><br></div></div></blockquote><div><br></div><d=
iv>From the sound of it, you want some kind of lazy IO, driven/generated by=
 a state monad. =A0Check out the &quot;safe-lazy-io&quot;. =A0I&#39;ve neve=
r used it, but the announcement is pretty convincing. <a href=3D"http://www=
.haskell.org/pipermail/haskell/2009-March/021133.html">http://www.haskell.o=
rg/pipermail/haskell/2009-March/021133.html</a></div>
</div>

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