[Haskell-beginners] explaining effects

[Imants Cekusins]

> What is the exact defintion of "effect".

let's try:

effect:
A change which is a consequence of an action (in this case, function call)

side effect:
change of environment state which is a consequence of an action (function call)

pure function:
calling this function does not affect environment state
function returns a value, that's all

I am not sure if function running inside e.g. state monad and
modifying this monad's state is pure, i.e. if state monad is
environment