Add intercalate to Data.List (ticket #971)

[haskell-directory.git] / Data / List.hs

diff --git a/Data/List.hs b/Data/List.hs
index da6c878..b006b29 100644 (file)
--- a/Data/List.hs
+++ b/Data/List.hs
@@ -35,6 +35,7 @@ module Data.List
    , reverse           -- :: [a] -> [a]
 
    , intersperse       -- :: a -> [a] -> [a]
+   , intercalate       -- :: [a] -> [[a]] -> [a]
    , transpose         -- :: [[a]] -> [[a]]
 
    -- * Reducing lists (folds)
@@ -100,6 +101,7 @@ module Data.List
    -- ** Predicates
    , isPrefixOf        -- :: (Eq a) => [a] -> [a] -> Bool
    , isSuffixOf        -- :: (Eq a) => [a] -> [a] -> Bool
+   , isInfixOf         -- :: (Eq a) => [a] -> [a] -> Bool
 
    -- * Searching lists
 
@@ -266,6 +268,17 @@ isPrefixOf (x:xs) (y:ys)=  x == y && isPrefixOf xs ys
 isSuffixOf              :: (Eq a) => [a] -> [a] -> Bool
 isSuffixOf x y          =  reverse x `isPrefixOf` reverse y
 
+-- | The 'isInfixOf' function takes two lists and returns 'True'
+-- iff the first list is contained, wholly and intact,
+-- anywhere within the second.
+--
+-- Example:
+--
+-- >isInfixOf "Haskell" "I really like Haskell." -> True
+-- >isInfixOf "Ial" "I really like Haskell." -> False
+isInfixOf               :: (Eq a) => [a] -> [a] -> Bool
+isInfixOf needle haystack = any (isPrefixOf needle) (tails haystack)
+
 -- | The 'nub' function removes duplicate elements from a list.
 -- In particular, it keeps only the first occurrence of each element.
 -- (The name 'nub' means \`essence\'.)
@@ -384,6 +397,12 @@ intersperse _   []      = []
 intersperse _   [x]     = [x]
 intersperse sep (x:xs)  = x : sep : intersperse sep xs
 
+-- | 'intercalate' @xs xss@ is equivalent to @('concat' ('intersperse' xs xss))@.
+-- It inserts the list @xs@ in between the lists in @xss@ and concatenates the
+-- result.
+intercalate :: [a] -> [[a]] -> [a]
+intercalate xs xss = concat (intersperse xs xss)
+
 -- | The 'transpose' function transposes the rows and columns of its argument.
 -- For example,
 --