[Haskell-cafe] One-shot? (was: Global variables and stuff)

Keean Schupke k.schupke at imperial.ac.uk
Thu Nov 11 15:14:24 EST 2004

I think you are right... The only safe operation I can see for a 
one-time init
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 ()


Judah Jacobson wrote:

>On Thu, 11 Nov 2004 12:27:17 +0000, Ben Rudiak-Gould
><benjamin.rudiak-gould at cl.cam.ac.uk> 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:
>    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?
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>Haskell-Cafe at haskell.org

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