[Haskell-cafe] Splitting a string into chunks

[Sebastian Sylvan]

On 1/13/06, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
> On 1/13/06, Adam Turoff <adam.turoff at gmail.com> wrote:
> > Hi,
> >
> > I'm trying to split a string into a list of substrings, where substrings
> > are delimited by blank lines.
> >
> > This feels like it *should* be a primitive operation, but I can't seem
> > to find one that works.  It's neither a fold nor a partition, since each
> > chunk is separated by a 2-character sequence.  It's also not a grouping
> > operation, since ghc's Data.List.groupBy examines the first element in a
> > sequence with each candidate member of the same sequence, as
> > demonstrated by:
> >
> >     Prelude> :module + Data.List
> >     Prelude Data.List> let t = "asdfjkl;"
> >     Prelude Data.List> groupBy (\a _ -> a == 's') t
> >     ["a","sdfjkl;"]
> >
> > As a result, I've wound up with this:
> >
> >     -- Convert a file into blocks separated by blank lines (two
> >     -- consecutive \n characters.) NB: Requires UNIX linefeeds
> >
> >     blocks :: String -> [String]
> >     blocks s = f "" s
> >       where
> >         f "" [] = []
> >         f s [] = [s]
> >         f s ('\n':'\n':rest) = (s:f "" rest)
> >         f s (a:rest) = f (s ++ [a]) rest
> >
> > Which somehow feels ugly.  This feels like it should be a fold, a group
> > or something, where the test is something like:
> >
> >     (\a b -> (a /= '\n') && (b /= '\n'))
>
> Off the top of my head:
>
> blocks = map concat . groupBy (const null) . lines
>
> The lines function splits it into lines, the groupBy will group the
> list into lists of lists and split when the sedond of two adjacent
> elements is null (which is what an empty line passed to lines will
> give you) and then a concat on each of the elements of this list will
> "undo" the redundant lines-splitting that lines performed...
>

Sorry, I got the meaning of groupBy mixed up, it should be

blocks = map concat . groupBy (const (not . null)) . lines

/S

--
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862