[switching to Haskell-cafe] At 19:37 23/12/03 +0100, Tomasz Zielonka wrote:

On Tue, Dec 23, 2003 at 05:26:20PM +0000, Graham Klyne wrote:

...

[1] http://www.ninebynine.org/Software/Learning-Haskell-Notes.html

Thanks, that was a nice reading :)

Thanks! (If by any chance there's anything here that might be useful for the Wiki, anyone may feel free to plunder it.)

I have some comments:

8. Your explanation of Functor excludes many useful Functors which are rather not collections. For example, every monad (like IO) can be a Functor if you take fmap = Monad.liftM.

For [] and Maybe this would give the same operation as in their normal instances.

That's an interesting perspective that I wasn't aware of... I need to think about that. Meanwhile, I've added your observation to my notes. [later] 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?

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", right?

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. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact