List Monad

As we already mentioned in the Applicatives section, the underlying effect of the List type is support for non-deterministic computations. Maybe computations can return either Nothing or a Just value, while List computations can return zero, one or multiple values based on their length. Let's see how this is defined in the Monad instance of List:

instance Monad [] where
   -- return :: a -> [a]
   return x = [x]
   -- (>>=)  :: [a] -> (a -> [b]) -> [b]
   m >>= f  = [y | x <- m, y <- f x]

The bind operator in this case is defined using list comprehension. Can you define it using the functions map and concat?

The return method simply takes a value x and puts it into a List structure [x]. The (>>=) method for lists extracts all the values x from the list m and applies the function f to each of them, combining all the results in one final list. As with the Maybe monad, the bind operator, allows us to chain operations together with lists as well. With lists, these chaining operations will combine all output possibilities in a single result list.

Let's take a look at a simple example of chaining list operations together. Imagine we want to model mitosis, a process where a single cell divides into two identical daughter cells. First, we create a function mitosis that simply splits a cell in two:

mitosis :: String -> [String] -- we will represent cells with simple strings
mitosis = replicate 2

ghci> ["Cell"] >>= mitosis
["Cell", "Cell"]

With the monadic instance of lists, we have a simple way of chaining multiple operations on lists. We can chain the result of multiple cell replications starting with one cell into one final list:

ghci> ["Cell"] >>= mitosis >>= mitosis >>= mitosis
["Cell", "Cell", "Cell", "Cell", "Cell", "Cell", "Cell", "Cell"]

In this case, the mitosis function is applied to each element of the list, providing a resulting list to be passed to the subsequent functions.

The monadic instance of lists also allows us to use the do notation:

threeGens :: String -> [String]
threeGens gen0 = do
        gen1 <- mitosis gen0
        gen2 <- mitosis gen1
        gen3 <- mitosis gen2
        return gen3
ghci> threeGens "Cell"
["Cell", "Cell", "Cell", "Cell", "Cell", "Cell", "Cell", "Cell"]

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