[pwnstigator]

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 ().

[jrockway]

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.

[anc2020]

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 :)