Adding zeros between elements in list?

Question

I'm trying to change a list in haskell to include 0 between every element. If we have initial list [1..20] then i would like to change it to [1,0,2,0,3..20]

What i thought about doing is actually using map on every function, extracting element then adding it to list and use ++[0] to it but not sure if this is the right approach or not. Still learning haskell so might have errors.

My code:

x = map classify[1..20] 

classify :: Int -> Int 
addingFunction 0 [Int]


addingFunction :: Int -> [a] -> [a]
addingFunction x xs = [a] ++ x ++ xs 

Answer 1

intersperse is made for this. Just import Data.List (intersperse), then intersperse 0 yourList.

Answer 2

You cannot do this with map. One of the fundamental properties of map is that its output will always have exactly as many items as its input, because each output element corresponds to one input, and vice versa.

There is a related tool with the necessary power, though:

concatMap :: (a -> [b]) -> [a] -> [b]

This way, each input item can produce zero or more output items. You can use this to build the function you wanted:

between :: a -> [a] -> [a]
sep `between` xs = drop 1 . concatMap insert $ xs
  where insert x = [sep, x]

0 `between` [1..10]
[1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,0,10]

Or a more concise definition of between:

between sep = drop 1 . concatMap ((sep :) . pure)