{- Copyright (C) 2010 Dr. Alistair Ward This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. -} {- | [@AUTHOR@] Dr. Alistair Ward [@DESCRIPTION@] -} module ToolShed.Data.List.Splits( -- * Types -- ** Type-synonyms -- Split, -- * Functions -- splitsFrom, splitsLeftFrom, splitsRightFrom ) where -- | The polymorphic pair, resulting from splitting a list of arbitrary type. type Split a = ([a] {-left list-}, [a] {-right list-}) -- | Use the specified transformation, to generate a list of 'Split's, from the initial one. splitsFrom :: (Split a -> Split a) -- ^ The function used to transform one /split/ into the next. -> Int -- ^ Index. -> [a] -- ^ The polymorphic input list from which the /splits/ are generated. -> [Split a] -- ^ The list of all required splits of the single input list. splitsFrom transformation i | i < 0 = error $ "ToolShed.Data.List.Splits.splitsFrom:\tnegative starting-index; " ++ show i | otherwise = iterate transformation . splitAt i {- | * Create the set of all 'Split's, migrating left from the specified location. * CAVEAT: 'init' fails when 'fst' has been reduced to null. -} splitsLeftFrom :: Int -- ^ Index. -> [a] -- ^ The polymorphic input list from which the /splits/ are generated, as the index is stepped left -> [Split a] -- ^ The list of all required splits of the single input list. splitsLeftFrom = splitsFrom (\(l, r) -> (init l, last l : r)) {- | * Create the set of all 'Split's, migrating right from the specified location. * CAVEAT: pattern-match against @ : @ fails, when 'snd' has been reduced to 'null'. -} splitsRightFrom :: Int -- ^ Index. -> [a] -- ^ The polymorphic input list from which the /splits/ are generated, as the index is stepped right. -> [Split a] -- ^ The list of all required splits of the single input list. splitsRightFrom = splitsFrom (\(l, r : rs) -> (l ++ [r], rs))