[Haskell-cafe] global variables

[Isaac Dupree]

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Adrian Hey wrote:
> Furthermore, if it is going to take this state handle as an explicit
> argument then you need to provide some way for users to get this
> state handle. This could be by..
>  1 - Making it an argument of main.
>  2 - Exposing a newLibWhateverState constructor
>  3 - Exposing a getLibWhateverState "getter".
> 
> Problems..
>  1 Requires the type of main to depend on what IO libs are used.
>    Also the Boot code that invokes main must get this state handle
>    from somewhere.
>  2 Potentially permits 2 or more libWhateverStates to be created
>    (in which case all bets are off re. the safety proprties I was
>     talking about).
>  3 Can't be implemented without making use of..you know what.

Making it an argument of main (1) is somewhat the same as doing (3) -
consider getArgs.  What bothers me is the IO-state in the Haskell
standard (arguments, random state...) which is a set not extensible
without the hack.  (It bothers me that there _are_ so many things in
that set, somewhat ad-hoc-ly it seems :)


> Unfortunately the situation seems worse than this. AFAICT we haven't
> even got a consensus that that there is a real problem here that needs
> any kind of solution :-)

The unsafePerformIO hack being used is not very satisfactory given how
many optimizations make it difficult to use safely in practice.  This
hack is also used many places.  I would be happier if that situation
were not true, and I suspect there's something like a consensus on
_that_. (maybe not as strong as "_needs_ a solution" in the short-to-mid
term future)

Isaac


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