Bardur Arantsson comments the comment of Joe Quinn:

> On 8/7/2013 11:00 AM, David Thomas wrote:

>> twice :: IO () -> IO ()
>> twice x = x >> x
>>
>> I would call that evaluating x twice (incidentally creating two
>> separate evaluations of one pure action description), but I'd like to
>> better see your perspective here.

> 
> x is only evaluated once, but /executed/ twice. For IO, that means
> magic. For other types, it means different things. For Identity, twice =
> id!
>

Your point being? x is the same thing regardless of how many times you
run it.

What do you mean by "the same thing"? You cannot compare 'them' in any reasonable sense.

This, the impossibility to check putStr "c" == putStr "c", is btw, a refutation of the claim by Tom Ellis that you can do even less with (). The void object is an instance of the Eq and Ord classes. And of Show as well.

You make the distinction between "evaluate", and "execute" or "run", etc. This is not functional. Your program doesn't "run" anything, it applies (>>=) (or equivalent) to an IO (...) object. This is the only "practical evaluation" of it, otherwise it can be passed (or duplicated as above). But you cannot apply "bind" twice to the same instance of it (in fact, as I said above, "the same instance" is a bit suspicious concept...).

The "running" or "execution" takes place outside of your program. In such a way Richard O'Keefe and I converge... That's why I say that the concept of purity is meaningless in the discussed context. It is a kind of counterfeit notion, inherited from "pure functions" to something which belongs to two different worlds.

Jerzy Karczmarczuk

PS. I believe that some impure remarks about the familiarity of X or Y with English do not belong to this forum.