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?
Subtle Hint:
Though there is only one kind of the chain, it comes in various types. Three types are mentioned in the text.
Moderate Hint:
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.
Decisive Hint:
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'.