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POCan parser combinators be made efficient?
 

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  1. POCan parser combinators be made efficient?
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    <p>Around 6 years ago, I benchmarked my own parser combinators in OCaml and found that they were ~5&#215; slower than the parser generators on offer at the time. I recently revisited this subject and benchmarked Haskell's Parsec vs a simple hand-rolled <a href="http://www.engr.mun.ca/~theo/Misc/exp_parsing.htm" rel="noreferrer">precedence climbing parser</a> written in F# and was surprised to find the F# to be 25&#215; faster than the Haskell.</p> <p>Here's the Haskell code I used to read a large mathematical expression from file, parse and evaluate it:</p> <pre><code>import Control.Applicative import Text.Parsec hiding ((&lt;|&gt;)) expr = chainl1 term ((+) &lt;$ char '+' &lt;|&gt; (-) &lt;$ char '-') term = chainl1 fact ((*) &lt;$ char '*' &lt;|&gt; div &lt;$ char '/') fact = read &lt;$&gt; many1 digit &lt;|&gt; char '(' *&gt; expr &lt;* char ')' eval :: String -&gt; Int eval = either (error . show) id . parse expr "" . filter (/= ' ') main :: IO () main = do file &lt;- readFile "expr" putStr $ show $ eval file putStr "\n" </code></pre> <p>and here's my self-contained precedence climbing parser in F#:</p> <pre><code>let rec (|Expr|) = function | P(f, xs) -&gt; Expr(loop (' ', f, xs)) | xs -&gt; invalidArg "Expr" (sprintf "%A" xs) and loop = function | ' ' as oop, f, ('+' | '-' as op)::P(g, xs) | (' ' | '+' | '-' as oop), f, ('*' | '/' as op)::P(g, xs) -&gt; let h, xs = loop (op, g, xs) match op with | '+' -&gt; (+) | '-' -&gt; (-) | '*' -&gt; (*) | '/' | _ -&gt; (/) |&gt; fun op -&gt; loop (oop, op f h, xs) | _, f, xs -&gt; f, xs and (|P|_|) = function | '('::Expr(f, ')'::xs) -&gt; Some(P(f, xs)) | c::_ as xs when '0' &lt;= c &amp;&amp; c &lt;= '9' -&gt; let rec loop n = function | c2::xs when '0' &lt;= c2 &amp;&amp; c2 &lt;= '9' -&gt; loop (10*n + int(string c2)) xs | xs -&gt; Some(P(n, xs)) loop 0 xs | _ -&gt; None </code></pre> <p>My impression is that even state-of-the-art parser combinators waste a lot of time back tracking. Is that correct? If so, is it possible to write parser combinators that generate state machines to obtain competitive performance or is it necessary to use code generation?</p> <p><strong>EDIT:</strong></p> <p>Here's the OCaml script I used to generate a ~2Mb expression for benchmarking:</p> <pre><code>open Printf let rec f ff n = if n=0 then fprintf ff "1" else fprintf ff "%a+%a*(%a-%a)" f (n-1) f (n-1) f (n-1) f (n-1) let () = let n = try int_of_string Sys.argv.(1) with _ -&gt; 3 in fprintf stdout "%a\n" f n </code></pre>
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    1. USJon Harrop
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    1. COI've always been under the impression that parser combinators were inefficient, but you'd really have to try both solutions in the same language to get a good gauge of the speed difference.
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    2. COit seems you're using parsec 3.x, which according to this is slower than parsec 2 http://www.mail-archive.com/haskell-cafe@haskell.org/msg81686.html
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      1. USMauricio Scheffer
    3. COAn issue is possibly also, that you're using char lists in Haskell. Parsing a Bytestring is faster.
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