is type IO (). All results have to be returned via side effects. Hence with
my named-MVar proposal the first execution of the init function initialises
certain named-MVars, and subsequent executions do nothing at all. The
functions in the library would use the names-MVars directly. Therefore the
once function in the NamedSem library is:
once :: IO () -> IO ()
Keean.
Judah Jacobson wrote:
On Thu, 11 Nov 2004 12:27:17 +0000, Ben Rudiak-Gould
<[EMAIL PROTECTED]> 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.
Reflecting on the matter, I don't think that oncePerString is type-safe. For example, it allows us to create the following:
ref :: IO (IORef a) ref = oncePerString "foo" (newIORef undefined)
Here's an example in which we subvert the type system (and probably crash the program) by writing a String and reading an Int from the same IORef:
do ref >>= writeIORef ("foo")
(x :: Int) <- ref >>= readIORef print x
This is similar to the reason for ML's value monomorphism restriction. In contrast, oncePerType preserves monomorphism nicely, since all instances of Typeable are monomorphic.
Thoughts? Am I missing something?
-Judah
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