[PaulAJ]

The "Monad" typeclass in Haskell is a unification of a bunch of things that are treated separately in other programming languages. By "unification" I mean the same kind of thing that physicists mean when they say that Maxwell unified light and electrical phenomena, or that Newton unified physics and astronomy.

It is this unification that makes monads so powerful; one abstraction handles sequencing, exceptions, non-determinism, parsing, IO, transactions, asynchronous state machines and tons of other stuff.

[chii]

i like to explain monads as the same sort of thinking process as a 'design pattern' in OOP. People who have done OOP would have heard of things like the visitor pattern. In OOP, patterns aren't unified in some class, but could be.

Monads (and type classes in general) are like design patterns in OOP, in that they encode a common use case. Except in haskall, you can encode this pattern into the language semantics.