[Haskell-cafe] Doing without IORef

[Claude Heiland-Allen]

Jinwoo Lee wrote:
> Hi,
> 
> Recently I wrote a code that uses readline library 
> (System.Console.Readline).
> I had to maintain a state (file path) and do IO throughout the code, so 
> I decided to use StateT monad.
> 
> The problem was that in order to retrieve the current state (file path) 
> inside the handler that had been registered by using bindKey function of 
> readline, I had to resort back to using IORef rather than using the 
> state stored in the StateT monad. It's because the handler for bindKey 
> should have the type of Int -> Char -> IO Int.

[snip]

> Is there any way in which I can do without IORef in tabHandler and 
> commandLoop (written in red and bold, if you can see)?

Probably not, but this is worth a read if you want to hide the plumbing 
behind the scenes a bit:

"Monadic tunnelling: the art of threading one monad through another"
http://www.haskell.org/pipermail/haskell-cafe/2007-July/028501.html


Claude
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