I'm not very good in understanding the difference between the imperative and declarative programming paradigms. I read about Haskell being a declarative language. To an extend I would says yes but there is a something that bothers me in respect to the definition of imperative.
When I have a datastruct and use Haskell's functions to transform that I actually just told WHAT to transform. So I give the datastruct as argument to a function and am happy with the result.
But what if there is no function that actually satisfies my needs?
I would start writing an own function which expects the datastruct as an argument. After that I would start writing how the datastruct should be processed. Since I only can call native Haskell functions I'm still with the declarative paradigm right? But what when I start using an "if statement". Wouldn't that end the declarative nature since I'm about to tell the program HOW to do stuff from that point?
Perhaps this is a matter of perspective. The way I see it, there is nothing imperative about defining things in terms of other things because we can always replace something with its definition (as long as the definition is pure). That is, if we have f x = x + 1, then any place we see f z we can replace with z + 1. So pure functions shouldn't really be considered instructions; they should be considered definitions.
A lot of Haskell code is considered declarative for this reason. We simply define things as (pure) functions of other things.
It is possible to write imperative-style code in Haskell as well. Sometimes we really do want to say something like "do A, then do B, then do C". This adds a new dimension to the simple world of function application: we need a notion of 'happens before'. Haskell has embraced the Monad concept to describe computations that have an order of evaluation. This turns out to be very handy because it can encapsulate effects such as changing state.