Perspectives on learning and using Haskell
t.zielonka at students.mimuw.edu.pl
Wed Dec 24 16:22:46 EST 2003
On Wed, Dec 24, 2003 at 10:39:33AM +0000, Graham Klyne wrote:
> It now seems to me that (some?) Monads are kinds of Functors, generalized
> to handle the "no value" case, and also composition.
> This also had me thinking about sequence: is there a generalization to
> arbitrary monads that rearranges the monadic structure?
Perhaps looking into Category Theory could be enlightening for both of
us - terms Monad and Functor where both taken from there.
> >11 and 18.
> > If you define an instance of Monad for ((->) e) then
> > return (putStrLn "Hello!") 'x'
> > is a proper IO () value. Probably still not sensible ;)
> Ah, I think I see your point. It would apply where monads are "nested",
> > Special treatment of 'return' could be helpful, but I am afraid that
> > it could also make it look special, like a return keyword in C.
> I certainly wouldn't argue for special treatment _in the language_, but
> OTOH, I think it might be helpful if compiler diagnostics hinted at the
> possibility when a type error is detected in a form like return x y.
.signature: Too many levels of symbolic links
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