On Thursday 11 Nov 2004 12:27 pm, Ben Rudiak-Gould wrote:
On the other hand, these are perfectly safe:
once' :: IO a -> IO (IO a) oncePerString :: String -> IO a -> IO a oncePerType :: Typeable a => IO a -> IO a
once' seems virtually useless unless you have top-level <-, but the other two don't need it. I'm not sure which would be preferable. I lean toward oncePerString as more flexible and predictable, though it requires a certain discipline on the part of its users.
Having taken a bit of time to look at this, I have to say that IMO saying they are "perfectly safe" is over stating things a bit :-) The only one that is perfectly safe is the first, but as you say, is useless (at least for the purposes under discussion) without the top-level <- extension. AFAICS the other two are unsound hacks. So it seems to me that either the top-level <- extension (in one form or another) really is necessary, or that top level TWI's are unnecessary. The latter is probably true, in a strict technical sense. But I can't see a way to live without them and keep modularity. In any case, I don't think there's any reason to force programmers "wear the hair shirt" in this respect (other than dogma and superstitious fear about the evils of "global variables" :-) Regards -- Adrian Hey