I have a chain.
I can extend this chain indefinitely.
For beginners, this chain is hard to understand.
They call it magical, yet it is not in reality.
I used to only declare.
But with this chain, I can finally command.
[Maybe, this chain is made from Io.]
What am I and what is this chain?
Though there is only one kind of the chain, it comes in various types. Three types are mentioned in the text.
The chain can be so many things. It can be a writer, it can be a reader, it can be a parser, it can store states, and many other things.
Moderate Hint 2:
There is a sugar. The chain can be done.
I am in seven letters. The chain is in five letters.
Decisive Hint 2:
I start with 'H'. The chain starts with 'M'.
Decisive Hint 3:
I end with 'l'. The chain ends with 'd'.