[Haskell-cafe] Splitting a string into chunks

Adam Turoff adam.turoff at gmail.com
Fri Jan 13 16:00:02 EST 2006


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

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
        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'))

Any thoughts?


-- Adam

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