[Haskell-cafe] Doing without IORef

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Thu Apr 3 11:58:13 EDT 2008

On Apr 3, 2008, at 6:03 , Jinwoo Lee wrote:
> 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.

You can hide the IORef inside the StateT... but this is one of  
several cases where I *really* wish the callbacks were MonadIO a  
=> ... -> a t.

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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