(Rewrite the last case, which made me notice a space leak in Data.ByteString, and an inefficient algorithm)
(Change Data.FastPackedString do Data.ByteString)
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Latest revision as of 15:59, 31 August 2009
|Haskell Performance Resource|
 1 Strings
Sometimes the cost of representing strings as lists of Char can be too much. In this case, you can instead use packed strings. There are a number of options:
- One of the packed string libraries, for example Data.ByteString
- Unboxed arrays of Word8 or Char
- Ptrs to foreign malloced Word8 buffers
The packed string libraries have the benefit over arrays of Word8 or Char types, in that they provide the usual list-like operations.
Some interesting results for Data.ByteString are documented here. In particular, it compares FPS against the existing PackedString and [Char] functions, and is used successfully with 1 terabyte strings.
 2 Example
Pete Chown asked the question:
I want to read a text file. As an example, let's use /usr/share/dict/words and try to print out the last line of the file.
The python version completes in around 0.05s.
 2.1 Attempt 1 : [Char]
import System.IO main = readFile "/usr/share/dict/words" >>= putStrLn.last.lines
Run in hugs, this program took several seconds to complete. Problem: interpreted (solution, use a Haskell compiler). Compiled, the program completes in a fairly quick 0.2s. Still, we can do better.
 2.2 Attempt 2 : Data.ByteString
Using ByteString, we get:
import qualified Data.ByteString as B import IO main = B.readFile "/usr/share/dict/words" >>= B.putStr . last . B.lines
Runs in 0.063s
 2.3 Attempt 3 : No Lists
Avoid splitting the file into lists at all, and just keep a single buffer (as a C programmer would perhaps do):
import qualified Data.ByteString as P import Maybe import IO main = P.readFile "/usr/share/dict/words" >>= P.putStrLn . snd . fromJust . P.breakLast '\n'
Runs in 0.013s
 2.4 Related work
An extended tutorial on using PackedStrings/ByteStrings for high performance string manipulating code is here.
A discussion of the fastest way to parse a file of numbers, comparing various approaches using ByteStrings.