Should we have primitive fill-once variables?

[Joachim Breitner]

Hi,

when reading the subject I was expecting something like this:

   -- | Creates an empty IVar
   newIVar :: IO (IVar a)

   -- | pure! but blocks until the IVar is written
   readIVar :: IVar a -> a 

   -- | tries to write to an IVar. 
   -- Succeeds if it is empty (returning True)
   -- Does nothing if it has been written to (returning False)
   writeIVar :: IVar a -> a -> IO Bool 

Alternatively:

   -- | all in one
   newIVar :: IO (a, a -> IO Bool)


Essentially a thunk, but with explicit control over filling it.
In fact, people have implemented something like this using C-- hacks
before: https://github.com/twanvl/unsafe-sequence

> This would make MonadFix's implementation much nicer, I think :)

This would suffice for MonadFix, right?

Sorry for derailing the thread :-)

Cheers,
Joachim

-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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