Lazy evaluation is not important item here. It is merely the semantics is defined in a way that the order of declarations does not matter.

Other languages have this property too - say, C# (not entirely true, but pretty close - at least if you think about top level methods etc.).

Also an example where the order of declarations does not matter at all, because there is just one declaration:

   x :: [Integer]
   x = 0 : 1 : x