[Haskell] Re: Global Variables and IO initializers
benjamin.franksen at bessy.de
Fri Nov 5 17:14:31 EST 2004
On Friday 05 November 2004 22:07, Keean Schupke wrote:
> So what
> we need is a way in the type system to tell the compiler the function
> must have
> a single unique definition... Something like:
> myRef :: Unique (IORef Int)
> myRef = uniquePerformIO $ newIORef 0
> and then have:
> runUnique :: Unique x -> x
In Eiffel it is called 'once' istead of 'Unique', e.g. (excuse my rusty
Eiffel, the syntax may be wrong)
once ref : Int
...routine body here...
Result := ...
The semantics is that the routine body is executed at most once, namely when
the feature is used for the first time.
Note that Eiffel allows arbitrary IO actions to be performed in the body of
once routines, just like in your Haskell example above. It is interesting to
note that the Eiffel community is quite aware of the problems this solution
has, i.e. that the procedure may have side-effects that happen at some
unpredictable moment in time -- especially when concurrent execution comes
into play. It is regarded as a matter of programmer discipline to ensure that
once routines do not have effects visible outside the class in which they are
Such an appeal to programmer discipline clearly fits not well with the spirit
of Haskell. I would argue that the actions to be performed inside such a
'once' or 'unique' initialization must be strictly limited to harmless ones
like allocation of reference cells. As i pointed out earlier, elements of a
commutative sub-monad of IO are not automatically harmless. How else can we
define "harmless" IO actions?
Maybe Ben Rudiak-Gould's idea to use (forall s . ST s) is teh right idea but I
still don't understand it...
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