Is there any intuition to understand join two functions in Monad?

Question

join is defined along with bind to flatten the combined data structure into single structure.

From type system view, (+) 7 :: Num a =>

Answer 1

An intuition about join is that is squashes 2 containers into one. .e.g

join [[1]] => [1]
join (Just (Just 1)) => 1
join (a christmas tree decorated with small cristmas tree) => a cristmas tree

etc ...

Now, how can you join functions ? In fact functions, can be seen as a container. If you look at a Hash table for example. You give a key and you get a value (or not). It's a function key -> value (or if you prefer key -> Maybe value). So how would you join 2 HashMap ?

Let's say I have (in python style) h={"a": {"a": 1, "b": 2}, "b" : {"a" : 10, "b" : 20 }} how can I join it, or if you prefer flatten it ? Given "a" which value should I get ? h["a"] gives me {"a":1, "b":2}. The only thing I can do with it is to find "a" again in this new value, which gives me 1. Therefore join h equals to {"a":1, "b":20}.

It's the same for a function.