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
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