Am Dienstag, 25. Januar 2005 00:29 schrieb Jorge Adriano Aires:
x = getLine >>= putStrLn This isn't obvious to me. So x is an action, and it does not always produces the same side effects when executed. But why should that make x/=x? It is the same action, it gets one line from the input, and then prints it...
OK, but then the different side-effects could not be used to distinguish putStrLn "hello" >> mzero and mzero. So I still believe, if you say these two are different, because they produce different output, you cannot easily insist on x === x.
A constant c :: a is just morphism(function) c : 0 -> a, where 0 is the initial object (empty set). ^^^^^^^^^ The empty set being an initial object means, for every a there is exactly one morphism from 0 to a. A constant is a function from a one-element-set to a.
This I don't agree with, I think you are using the word actions for two different things, the elements of type IO a, and their execution. What you
You're right, but one of my problems is to identify elements of type IO a. If the returned value isn't the thing, the execution must matter, but which parts of the execution are to be taken into account?
just showed is that those IO () elements (actions) when executed, always created different side effects in the real world. Not that the actions themselves are different.
But that is the problem, what does it mean for two actions to be the same? After all, writing "hello" to stdout is just a side-effect, like putting 4 on the stack and immediately ignoring it.
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