Haskell - replicate elements in a list

Question

I want to write a Haskell program that replicates the elements of a list a given number of times. Here's my code:

repli :: [a] -> a -> [a]
repli xs n = foldl1 (\x -> take n (repeat x)) xs

My problem is I get the following errors when compiling:

'take' is applied to too many arguments

couldn't match expected type '[a]->[a]' with actual type '[[a]]'

From your code, it seems that you want repli [1,2] 2 to generate [1,1,2,2], and not [1,2,1,2]. Is that right? Please clarify this in your question, since answers below sometimes chose the second interpretation. — chi

Mokosha Mokosha · Accepted Answer · 2014-11-26T15:24:55

The type signature for foldl1 is:

foldl1 :: (a -> a -> a) -> [a] -> a

Hence, the first argument needs to be a function of two arguments. The lambda expression you're passing only takes one argument. You probably meant to do something like this:

repli :: [a] -> Int -> [a]
repli xs n = concat $ map (\x -> take n (repeat x)) xs

Or, to do it better, you can use the replicate function:

repli :: [a] -> Int -> [a]
repli xs n = concat $ map (replicate n) xs

Or, to do it even betterer, you can use the [] monad:

repli :: [a] -> Int -> [a]
repli xs n = xs >>= replicate n

Haskell - replicate elements in a list

5 Answers