[Haskell-cafe] Reversing a string of words: C# v Perl V Ruby v Haskell

[Bernie Pope]

On 12/12/2006, at 11:13 AM, Brandon S. Allbery KF8NH wrote:

>
> On Dec 11, 2006, at 18:48 , Steve Downey wrote:
>
>> the typical good solution to this problem in c or c++ is to use a
>> string reverse function on the entire buffer, then re-reverse each
>> word. this leaves multiple spaces correctly embedded in the larger
>> string.
>> that approach, of course, won't work in haskell, since it relies on
>> updates. but if the challenge includes transforming "one two  three
>> four    " into "    four   three  two one",  how could this be done?
>
> Off the top of my head, the C++ one translates to:
>
> *Main> concatMap reverse . groupBy (\l r -> (l == ' ') == (r == '  
> ')) . reverse $ "one two  three   four    "
> "    four   three  two one"
>
> There are almost certainly more idiomatic ways to do it, but I'm  
> still learning.

Perhaps something like this is close to the algorithm you describe:

    import Data.Char (isSpace)

    rev :: String -> String
    rev str
       = revAcc (reverse str) []
       where
       revAcc [] acc = acc
       revAcc (x:xs) acc
          | isSpace x = acc ++ (x : revAcc xs [])
          | otherwise = revAcc xs (x : acc)

Cheers,
Bernie.