On Mon, 2008-01-07 at 18:15 +0300, Miguel Mitrofanov wrote:
> > data Stack a = Stack { run :: [a] -> (a, [a]) }
>
> [...skipped...]
>
> > But, I have simply no clue how to fix that. :-(
> > Can anybody give my a hint?
>
> Yes. It's simply impossible. The Stack data type can't be turned into
> a monad.
Indeed, the very first hint is that Stack isn't a functor because the
type variable a occurs both positively and negatively.
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