Re: An answer and a question to GHC implementors [was Re: How to make Claessen's Refs Ord-able?]

11 Apr 2002

      On Tue, Apr 09, 2002 at 11:06:14AM +0100, Simon Marlow wrote:
...
...
this usage of unsafePerformIO is such a staple of real-world Haskell
programming, it seems there should be some language (or experemental
compiler *wink wink ghc nudge*) support for it. I am not sure 
what form
it would take though.
<muse>
I did wonder once whether IO monad bindings should be allowed at the
top-level of a module, so you could say
module M where
     ref <- newIORef 42
wow. i really like this, I was thinking about something similar, but did
not want to have to introduce new syntax. using <- seems to make sense
here.
...
and the top-level IO would be executed as part of the module
initialization code.  This solves the problems with unsafePerformIO in a
cleanish way, but would add some extra complexity to implementations.
And I'm not sure what happens if one top-level IO action refers to other
top-level IO bindings (modules can be recursive, so you could get loops
too).
</muse>
...
getGlobalVar :: IO (IORef Int)
getGlobalVar = memoIO (newIORef 42)
note that this is not exactly the same since getting the global var is
in the io monad, but that really makes sense if you think 
about it. and
chances are you are already in IO if you need an IORef.
This doesn't really solve the problem we were trying to solve, namely
that passing around the IORef everywhere is annoying.  If we were happy
to pass it around all the time, then we would just say
main = do 
      ref <- newIORef 42
      ... pass ref around for ever ...
we wouldnt have to pass it around all the time with this scheme, you
would do something like

getGlobalVar :: IO (IORef Int)
getGlobalVar = memoIO (newIORef 42)

now you can use it anywhere as..

inc = do
	v <- getGlobalVar
	modifyIORef v (+ 1)

here is my simple implementation of memoIO which seems to do the right
thing. (at least under ghc)

memoIO :: IO a -> IO a
memoIO ioa = do
    v <- readIORef var
    case v of 
	Just x -> return x
	Nothing -> do
	    x <- ioa
	    writeIORef var (Just x) 
	    return x 
     where
	var = unsafePerformIO $ newIORef Nothing

-- 
---------------------------------------------------------------------------
John Meacham - California Institute of Technology, Alum. - john@repetae.net
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