Hi all, I'm trying to write a untab function that would split a string on tabs and return a list. Code is here. import Data.List (break, unfoldr) import Data.Char (String) untab :: String -> [String] untab s = unfoldr untab' s untab' :: String -> Maybe (String, String) untab' s | s == "" = Nothing | otherwise = Just (h, ts) where (h, t:ts) = break (== '\t') s This code raises an exception when handling the last portion of the string. Break returns a ("something", "") and t:ts cannot match on "". I was wondering if there way a clean way of handling this last case without adding tons of code. Some kind of "idiomatic expression" ;-) Thanks, Olivier.
Olivier Boudry wrote:
Hi all,
I'm trying to write a untab function that would split a string on tabs and return a list. Code is here.
import Data.List (break, unfoldr) import Data.Char (String)
untab :: String -> [String] untab s = unfoldr untab' s
untab' :: String -> Maybe (String, String) untab' s | s == "" = Nothing | otherwise = Just (h, ts) where (h, t:ts) = break (== '\t') s
This code raises an exception when handling the last portion of the string. Break returns a ("something", "") and t:ts cannot match on "".
untab' [] = Nothing untab' s = Just (h , drop 1 t) where (h,t) = break (== '\t') s
Why not do something like this instead?
untab [] = []
untab xs = head : untab (drop 1 tail)
where (head, tail) = break (== '\t') xs
BTW, going the extra step through unfoldr seems unnecessary to me - is
there any special reason to prefer unfolds over simple recursive
functions here? (Of course, you do get rid of the explicit recursive
call to untab - but in turn you have to run it all through unfoldr..)
Another, more pointless way is:
untab [] = []
untab xs = uncurry (:) $ second (untab . drop 1) $ break (== '\t') xs
(needs the additional import of Control.Arrow to get second)
On 6/11/07, Jules Bean
Olivier Boudry wrote:
Hi all,
I'm trying to write a untab function that would split a string on tabs and return a list. Code is here.
import Data.List (break, unfoldr) import Data.Char (String)
untab :: String -> [String] untab s = unfoldr untab' s
untab' :: String -> Maybe (String, String) untab' s | s == "" = Nothing | otherwise = Just (h, ts) where (h, t:ts) = break (== '\t') s
This code raises an exception when handling the last portion of the string. Break returns a ("something", "") and t:ts cannot match on "".
untab' [] = Nothing untab' s = Just (h , drop 1 t) where (h,t) = break (== '\t') s _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (3)
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Jules Bean -
Olivier Boudry -
Simon Brenner