99 questions/Solutions/14
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(*) Duplicate the elements of a list.
dupli [] = []
dupli (x:xs) = x:x:dupli xs
or, using list comprehension syntax:
dupli list = concat [[x,x] | x <- list]
or, using the list monad:
dupli xs = xs >>= (\x -> [x,x])
or, using concatMap:
dupli = concatMap (\x -> [x,x])
also using concatMap:
dupli = concatMap (replicate 2)
or, using foldl:
dupli = foldl (\acc x -> acc ++ [x,x]) []
or, using foldr:
dupli = foldr (\ x xs -> x : x : xs) []
or, using silliness:
dupli = foldr (\x -> ((x:) . (x:)) []