Am Sonntag, 13. Januar 2008 01:47 schrieb Brian Hurt:
So, I've been playing around with what I call the trivial monad:
module TrivialMonad where
data TrivialMonad a = M a
recover :: TrivialMonad a -> a recover (M x) = x
instance Monad TrivialMonad where
(M x) >>= f = f x (M x) >> f = f return x = M x fail s = undefined
This is actually a surprisingly usefull little sucker- it allows you to "demonadify" code. Which is usefull when you have some places where you want to use the code within a monad, and some places where you don't. You write the base bit of code monadically, and then demonadify it as needed.
The first question I have is it is possible to implement this guy without wrapping the value in a constructor? What I'd like to do is replace the: data TrivialMonad a = M a with something like: type TrivialMonad a = a and then be able to junk the recover function.
No, but you can use a newtype instead of data, so you have no run-time overhead.
The second question I have is: is there any hope of getting something like this into the standard library?
It's already there: Control.Monad.Identity
Thanks, Brian
Cheers, Daniel