[Haskell-cafe] A round of golf

[Creighton Hogg]

On Thu, Sep 18, 2008 at 1:29 PM, Don Stewart <dons at galois.com> wrote:
> wchogg:
>> Hey Haskell,
>> So for a fairly inane reason, I ended up taking a couple of minutes
>> and writing a program that would spit out, to the console, the number
>> of lines in a file.  Off the top of my head, I came up with this which
>> worked fine with files that had 100k lines:
>>
>> main = do
>>  path <- liftM head $ getArgs
>>  h <- openFile path ReadMode
>>  n <- execStateT (countLines h) 0
>>  print n
>>
>> untilM :: Monad m => (a -> m Bool) -> (a -> m ()) -> a -> m ()
>> untilM cond action val = do
>>  truthy <- cond val
>>  if truthy then return () else action val >> (untilM cond action val)
>>
>> countLines :: Handle -> StateT Int IO ()
>> countLines = untilM (\h -> lift $ hIsEOF h) (\h -> do
>>                                                 lift $ hGetLine h
>>                                                 modify (+1))
>>
>> If this makes anyone cringe or cry "you're doing it wrong", I'd
>> actually like to hear it.  I never really share my projects, so I
>> don't know how idiosyncratic my style is.
>
> This makes me cry.
>
>    import System.Environment
>    import qualified Data.ByteString.Lazy.Char8 as B
>
>    main = do
>        [f] <- getArgs
>        s   <- B.readFile f
>        print (B.count '\n' s)
>
> Compile it.
>
>    $ ghc -O2 --make A.hs
>
>    $ time ./A /usr/share/dict/words
>    52848
>    ./A /usr/share/dict/words 0.00s user 0.00s system 93% cpu 0.007 total
>
> Against standard tools:
>
>    $ time wc -l /usr/share/dict/words
>    52848 /usr/share/dict/words
>    wc -l /usr/share/dict/words 0.01s user 0.00s system 88% cpu 0.008 total

So both you & Bryan do essentially the same thing and of course both
versions are far better than mine.  So the purpose of using the Lazy
version of ByteString was so that the file is only incrementally
loaded by readFile as count is processing?