Graham Klyne wrote:
Wouldn't it be easier to simply define "once" as a common Haskell library function?
Depends on the type and the expected semantics. As Adrian Hey already pointed out, (once :: IO a -> IO a) with the obvious semantics is never going to be safe, because it's just not the case that
x = once (newIORef ()) y = x
has the same intended meaning as
x = once (newIORef ()) y = once (newIORef ())
No amount of compiler-specific magic is going to fix this.
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.
In any case there would need to be support for different scopes:
perProcess :: String -> IO a -> IO a perThread :: String -> IO a -> IO a perMachine :: String -> IO a -> IO a
I suppose you could add
perType :: Typeable a => IO a -> IO a
with the stipulation that types in different processes are distinct (which seems like the only safe assumption).
-- Ben
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