Where prelude alternatives/ classes and types to get values?

[Marc Weber]

> m would have to be IO, c would be a bit pointless - so IO () is a
> better return type.
It doesn't have to be IO. It can also be implemented using liftIO . modify .. Then you can use htis modify function from any monad beeing an instance of IOMonad..
> 
> You can then Hoogle for it: IORef a -> (a -> a) -> IO ()
> 
> http://haskell.org/hoogle/?q=IORef%20a%20-%3E%20(a%20-%3E%20a)%20-%3E%20IO%20()
> 
> 
> ... = do (New count) <- modify ioCounterRef (+1)
> 
> = do newcount <- modifyIORef ioCounterRef (+1)
> 
> It already exists :)
Hi Neil. 
Thanks for your hint. This time I already have found modifyIORef ;)
But as hoogle states:

Searched for modifyIORef	Results 1 - 2 of 2
Data.IORef.	modifyIORef	:: IORef a -> (a -> a) -> IO ()
Data.IORef.	atomicModifyIORef	:: IORef a -> (a -> (a, b)) -> IO b

the return type of modifyIORef is IO (), not IO <new value>
So only using this function wouldn't solve the problem, right?

Marc