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    <p>You could try using imperative hash maps from the hashtables package: <a href="http://hackage.haskell.org/package/hashtables" rel="nofollow noreferrer">http://hackage.haskell.org/package/hashtables</a> I remember I once got a moderate speedup compared to Data.Map. I wouldn't expect anything spectacular though.</p> <p><strong>UPDATE</strong></p> <p>I simplified your python code so I could test it on a single big file (100 million lines):</p> <pre><code>import sys histogram={} for word in sys.stdin.readlines(): if word in histogram: histogram[word] = histogram[word]+1 else: histogram[word] = 1 print histogram.get("the") </code></pre> <p>Takes 6.06 seconds</p> <p>Haskell translation using hashtables:</p> <pre><code>{-# LANGUAGE OverloadedStrings #-} import qualified Data.ByteString.Char8 as T import qualified Data.HashTable.IO as HT main = do ls &lt;- T.lines `fmap` T.getContents h &lt;- HT.new :: IO (HT.BasicHashTable T.ByteString Int) flip mapM_ ls $ \w -&gt; do r &lt;- HT.lookup h w case r of Nothing -&gt; HT.insert h w (1::Int) Just c -&gt; HT.insert h w (c+1) HT.lookup h "the" &gt;&gt;= print </code></pre> <p>Run with a large allocation area: <code>histogram +RTS -A500M</code> Takes 9.3 seconds, with 2.4% GC. Still quite a bit slower than Python but not too bad.</p> <p>According to the <a href="http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/runtime-control.html#rts-opts-compile-time" rel="nofollow noreferrer">GHC user guide</a>, you can change the RTS options while compiling: </p> <blockquote> <p>GHC lets you change the default RTS options for a program at compile time, using the -with-rtsopts flag (Section 4.12.6, “Options affecting linking”). A common use for this is to give your program a default heap and/or stack size that is greater than the default. For example, to set -H128m -K64m, link with -with-rtsopts="-H128m -K64m".</p> </blockquote>
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    1. POBuilding a histogram with haskell, many times slower than with python
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    1. COThank you,this got the time way down. After running the python version today, it takes ~22 seconds (don't know why it took over two minutes consistently yesterday), and haskell version does in 30 seconds with -A500M and 115 seconds without. Can I "embed" the -A500M to the binary somehow?
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    2. CO@Masse - compile your executable with the flag `-with-rtsopts=-A500M`
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    3. CO@Masse: updated my answer with compile-time RTS control
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