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    <p>I've come up with a Haskell solution that is 30× faster than the Haskell solution you posted (with my concocted test expression).</p> <p>Major changes:</p> <ol> <li>Change Parsec/String to Attoparsec/ByteString</li> <li>In the <code>fact</code> function, change <code>read</code> &amp; <code>many1 digit</code> to <code>decimal</code></li> <li>Made the <code>chainl1</code> recursion strict (remove $! for the lazier version).</li> </ol> <p>I tried to keep everything else you had as similar as possible.</p> <pre><code>import Control.Applicative import Data.Attoparsec import Data.Attoparsec.Char8 import qualified Data.ByteString.Char8 as B expr :: Parser Int expr = chainl1 term ((+) &lt;$ char '+' &lt;|&gt; (-) &lt;$ char '-') term :: Parser Int term = chainl1 fact ((*) &lt;$ char '*' &lt;|&gt; div &lt;$ char '/') fact :: Parser Int fact = decimal &lt;|&gt; char '(' *&gt; expr &lt;* char ')' eval :: B.ByteString -&gt; Int eval = either (error . show) id . eitherResult . parse expr . B.filter (/= ' ') chainl1 :: (Monad f, Alternative f) =&gt; f a -&gt; f (a -&gt; a -&gt; a) -&gt; f a chainl1 p op = p &gt;&gt;= rest where rest x = do f &lt;- op y &lt;- p rest $! (f x y) &lt;|&gt; pure x main :: IO () main = B.readFile "expr" &gt;&gt;= (print . eval) </code></pre> <p>I guess what I concluded from this is that the majority of the slowdown for the parser combinator was that it was sitting on an inefficient base, not that it was a parser combinator, per se.</p> <p>I imagine with more time and profiling this could go faster, as I stopped when I went past the 25× mark.</p> <p>I don't know if this would be faster than the precedence climbing parser ported to Haskell. Maybe that would be an interesting test?</p>
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    1. POCan parser combinators be made efficient?
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    1. USDarius Jahandarie
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    1. COParsec's default behavior of enabling backtracking is another reason why it's often slow. Parser combinator with the opposite behavior (e.g. uu-parsinglib) are often faster and simpler to reason about.
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      1. USJohn L
    2. COCan you show me your test expression?
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    3. CO+1 I can confirm that this is much faster and even faster than the F#. I'll convert the F# to use the equivalent of `ByteString` instead of `char` lists.
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      1. USJon Harrop
 

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