Hi Conal,
On Dec 9, 2007 6:09 PM, Conal Elliott
readIVar' :: IVar a -> a readIVar' = unsafePerformIO . readIVar
so, we do not need readIVar'. it could be a nice addition to the libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar".
The same argument applies any to pure function, doesn't it? For instance, a non-IO version of succ is unnecessary. My question is why make readIVar a blocking IO action rather than a blocking pure value, considering that it always returns the same value?
From the rest of Marc's post, I understood the point to be that readIVar lets you do something that readIVar' does not let you do: block until the IVar is written, then continue *without* first evaluating the thunk in the IVar to WHNF. I haven't used IVars myself, so this isn't informed by hands-on experience, but it does sound sensible to me that "block until the IVar has been written" and "evaluate the thunk to WHNF" should be separable.
All the best, - Benja