Please correct me if I'm wrong, but my understanding is that most Haskell code doesn't generate values, but "thunks" that evaluate to values if needed.
When you write
x = 4 + 5
you aren't setting x to 9, but creating a thunk that evaluates 4 + 5 at runtime. An Integer is a thunk that returns an integer and has no other effects. An IO Integer is one that does some I/O before returning that integer.
As I understand it, the only function that has the power to "do" anything (computation or IO) undernormal circumstances is main, which always has type IO ().
This is true, but not really relevant to the original article or the comment you are replying to.
When you sequence computations with >>=, like the grandparent does, you generally evaluate the left side of the operation before running the computation. That is the point of monads; sequencing computations and controlling the order of evaluation. Sine the rhs depends on the lhs, the sequence is "evaluate lhs completely", "evaluate rhs completely", and so on.
I think you are correct that the language does think of all expressions in terms of thunks, but I'm guessing `4 + 5` won't actually be run at run-time!
Since it's pure a decent compiler should be able to simply replace any `x` with the number 9 - correct me if I'm wrong :)
Any primitive (C code) however won't run until run-time, which is probably the pressing reason putStr doesn't run until run-time (and that's a good thing :)
When you write
you aren't setting x to 9, but creating a thunk that evaluates 4 + 5 at runtime. An Integer is a thunk that returns an integer and has no other effects. An IO Integer is one that does some I/O before returning that integer.As I understand it, the only function that has the power to "do" anything (computation or IO) undernormal circumstances is main, which always has type IO ().