Re: [Haskell-cafe] Relaxing atomicity of STM transactions

On Tue, Sep 28, 2010 at 8:54 AM, Felipe Lessa wrote:

On Tue, Sep 28, 2010 at 10:41 AM, Peter Robinson wrote:

...

readTVarIO :: TVar a -> IO a

One needs to know if it is ok to wrap this IO action into an STM action.  For example,

...

data I a = I a

looselyReadTVar :: TVar a -> STM a looselyReadTVar tvar =   let v = unsafePerformIO (I <$> readTVarIO tvar)   in case v of I x -> return x

The 'case' is needed because otherwise the TVar would be read only when its value was requested, and we want to keep the ordering.  The 'I' datatype is used to avoid evaluating the user's value (which could even be 'undefined').

Note that this function can be used on any monad, but I don't think that is a good idea =).

Cheers!

Isn't there an 'unsafeIOToSTM' function somewhere? Something like:

unsafeIOToSTM (IO k) = STM k

Then you might not need the case statement. Antoine