-{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, MagicHash #-}
+
-----------------------------------------------------------------------------
-- |
-- Module : Data.List
--
-- > inits "abc" == ["","a","ab","abc"]
--
+-- Note that 'inits' has the following strictness property:
+-- @inits _|_ = [] : _|_@
inits :: [a] -> [[a]]
-inits [] = [[]]
-inits (x:xs) = [[]] ++ map (x:) (inits xs)
+inits xs = [] : case xs of
+ [] -> []
+ x : xs' -> map (x :) (inits xs')
-- | The 'tails' function returns all final segments of the argument,
-- longest first. For example,
--
-- > tails "abc" == ["abc", "bc", "c",""]
--
+-- Note that 'tails' has the following strictness property:
+-- @tails _|_ = _|_ : _|_@
tails :: [a] -> [[a]]
-tails [] = [[]]
-tails xxs@(_:xs) = xxs : tails xs
-
+tails xs = xs : case xs of
+ [] -> []
+ _ : xs' -> tails xs'
-- | The 'subsequences' function returns the list of all subsequences of the argument.
--
-- characters. The resulting strings do not contain newlines.
lines :: String -> [String]
lines "" = []
+#ifdef __GLASGOW_HASKELL__
+-- Somehow GHC doesn't detect the selector thunks in the below code,
+-- so s' keeps a reference to the first line via the pair and we have
+-- a space leak (cf. #4334).
+-- So we need to make GHC see the selector thunks with a trick.
+lines s = cons (case break (== '\n') s of
+ (l, s') -> (l, case s' of
+ [] -> []
+ _:s'' -> lines s''))
+ where
+ cons ~(h, t) = h : t
+#else
lines s = let (l, s') = break (== '\n') s
in l : case s' of
[] -> []
(_:s'') -> lines s''
+#endif
-- | 'unlines' is an inverse operation to 'lines'.
-- It joins lines, after appending a terminating newline to each.