If Haskell allows you to express code as simple and correct at the expense of being fast, I think arguments can be made both ways as to whether or not that's a

If Haskell allows you to express code as simple and correct at the expense of being fast, I think arguments can be made both ways as to whether or not that's a bug. If we accept the premise that the first two are more important than the last, then I think it follows that it's not a bug. To your second point, I think the author is making an argument that it can perhaps even be earlier than that: the bug is introduced when the programmer's model of the behavior differs from reality. That is to say, the "bug" exists purely in the developer's mind. After that it's a matter of course for the bug to be transferred from there to the codebase. If the developer chooses to interrogate their model before committing it to code, there is the possibility of the bug being caught before it ever makes it into the codebase. But perhaps it is still correct to say the bug still existed, for a short time, at least.