I&#39;ve just recently started learning how to use the enumerator library. I&#39;m designing a utility that parses a file where each line is a JSON object. I&#39;ve designed it to look like:<div><br></div><div><div>source enumerator (enumHandle pretty much) -&gt; </div>
<div>chunk by lines (enumeratee) -&gt; </div><div>parse a line into an Object (enumeratee) -&gt; </div><div>filter objects based on a criteria (enumeratee) -&gt; </div><div>limit some keys from each object (enumeratee) -&gt;</div>
<div>encode the object into a lazy bytestring (enumeratee) -&gt;</div><div>output the file to stdout (iteratee)</div><div><br></div><div>I&#39;m having difficulties with the types, particularly composing the enumeratees in the middle. Someone in #haskell said that&#39;s a good case for &gt;=&gt; from Control.Monad but that seems to not like it if an enumeratee changes the type between the input and output.</div>
<div><br></div><div>Here&#39;s the relevant types:</div><div><br></div><div>type Object = Map Text Value</div><div><br></div><div><div>pipeline :: MonadIO m =&gt; (Enumerator ByteString IO ()) -&gt; [Text] -&gt; [Filter] -&gt; Iteratee a m ()</div>
</div><div><div>pipeline s rfs fs = s $$ splitLines    &gt;=&gt;</div><div>                         parseLine            &gt;=&gt;</div><div>                         (filterObjects fs)   &gt;=&gt;</div><div>                         (restrictFields rfs) &gt;=&gt;</div>
<div>                         encoder              $$</div><div>                         output</div></div><div><div>splitLines :: Monad m =&gt; Enumeratee ByteString ByteString m b</div></div><div><div>parseLine :: Monad m =&gt; Enumeratee ByteString Object m b</div>
</div><div><div>filterObjects :: Monad m =&gt; [Filter] -&gt; Enumeratee Object Object m b</div></div><div><div>restrictObjects :: Monad m =&gt; [Text] -&gt; Enumeratee Object Object m b</div></div><div><div>encoder :: Monad m =&gt; Enumeratee Object LBS.ByteString m b</div>
</div><div><div>output :: MonadIO m =&gt; Iteratee LBS.ByteString m ()</div></div><div><br></div><div>I&#39;m using &gt;=&gt; because I&#39;d prefer to compose them left-right for readability</div><div><br></div><div>Here&#39;s the error I get</div>
<div> </div><div><div> Couldn&#39;t match expected type `ByteString&#39;</div><div>                with actual type `M.Map Text Value&#39;</div><div>    Expected type: Data.Enumerator.Step ByteString m b1</div><div>                   -&gt; Iteratee ByteString m b2</div>
<div>      Actual type: Enumeratee ByteString Object m0 b0</div><div>    In the first argument of `(&gt;=&gt;)&#39;, namely `parseLine&#39;</div><div>    In the second argument of `(&gt;=&gt;)&#39;, namely</div><div>      `parseLine</div>
<div>     &gt;=&gt; (filterObjects fs) &gt;=&gt; (restrictFields rfs) &gt;=&gt; encoder&#39;</div></div><div><br></div><div>Any ideas?</div><div><div><br></div>-- <br>Michael Xavier<br><a href="http://www.michaelxavier.net" target="_blank">http://www.michaelxavier.net</a><div>
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