[Haskell-cafe] Reading files efficiently

Donald Bruce Stewart dons at cse.unsw.edu.au
Sun Mar 19 09:45:40 EST 2006

> I've got another n00b question, thanks for all the help you have been 
> giving me!
> 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. 
> First of all I came up with this program:
> import System.IO
> main = readFile "/usr/share/dict/words" >>= putStrLn.last.lines
> This program gives the following error, presumably because there is an 
> ISO-8859-1 character in the dictionary:
> "Program error: <handle>: IO.getContents: protocol error (invalid 
> character encoding)"
> How can I tell the Haskell system that it is to read ISO-8859-1 text 
> rather than UTF-8?
> I now used iconv to convert the file to UTF-8 and tried again.  This 
> time it worked, but it seems horribly inefficient -- Hugs took 2.8 
> seconds to read a 96,000 line file.  By contrast the equivalent Python 
> program:
> print open("words", "r").readlines()[-1]
> took 0.05 seconds.  I assume I must be doing something wrong here, and 
> somehow causing Haskell to use a particularly inefficient algorithm. 
> Can anyone give me any clues what I should be doing instead?

a) Compile your code with GHC instead of interpreting it. GHC is blazing fast.

    $ ghc -O A.hs
    $ time ./a.out
    ./a.out  0.23s user 0.01s system 91% cpu 0.257 total

b) If not satisifed with the result, Use packed strings (as python does).


    import qualified Data.FastPackedString as P
    import IO
    main = P.readFile "/usr/share/dict/words" >>= P.hPut stdout . last . P.lines

    $ ghc -O2 -package fps B.hs
    $ time ./a.out
    Zyzzogeton./a.out  0.04s user 0.02s system 86% cpu 0.063 total

0.06s is ok with me  :)

-- Don

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