[verttii]

I'm not talking about the compiler or the evaluation. From programmer's perspective the language does seem fairly declarative to me, mostly consisting of expressions instead of statements.

I don't know what's wrong about thinking about expressions as declarative which in my opinion they are. How would thinking about expressions imperatively help me avoid those exponential runtime costs?

For example consider the list comprehension: [toUpper c | c <- s]

If you compare this with your typical imperative for loop to construct the same data structure I find this declarative.