[Haskell-cafe] Re: Why is there no splitBy in the list module?
donn at drizzle.com
Tue Jul 11 03:20:35 EDT 2006
Quoth John Meacham <john at repetae.net>:
| just a note: I find splitBy a much nicer routine to provide.
|> splitBy :: (a -> Bool) -- ^ whether char is a separator
|> -> [a] -- ^ list to split
|> -> [[a]]
Starting from the assumption that in most cases the list to be split
will be [Char], so we can reasonably hold this up against string functions
in other languages -- I think split is commonly
split :: String -> String -> [String]
i.e., the separator is [a], not a. Of course next they'll want regular
expressions, but I use string separators of length > 1 a lot and rarely
bother with regexps.
But string splitBy might be an interesting alternative to regexp splits.
splitBy :: ([a] -> Maybe Int) -> [a] -> [[a]]
breakBy :: ([a] -> Maybe Int) -> [a] -> Maybe ([a], [a])
breakBy f s = by  f s
by _ _  = Nothing
by c f s@(a:ax) = case f s of
Just n -> Just ((reverse c), (drop n s))
Nothing -> by (a:c) f ax
splitBy f s = case breakBy f s of
Just (l, r) -> l:(splitBy f r)
Nothing -> [s]
Where in the simplest case the matching function would simply compare with
a separator string, but could easily do more - eat up trailing blanks,
Note use of Maybe, basically to disambiguate end of data with and without
Donn Cave, donn at drizzle.com
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