[Haskell-cafe] Space questions about intern and sets

Gracjan Polak gracjan at acchsh.com
Thu Jun 9 10:38:30 EDT 2005

Udo Stenzel wrote:
> Gracjan Polak wrote:
>>>>iorefset :: Ord a => IORef(Map.Map a a)
>>>>iorefset = unsafePerformIO $ do
>>>>    newIORef $ Map.empty
>>I could have as many dictionaries as there are types. The problem is I 
>>get one dictionary for each object which defeats the idea.
> I believe the (Ord a) constraint acts like a function argument.
> Therefore iorefset is no CAF, cannot be memoized itself and you get one
> dictionary per invocation.  On the other hand, that is what is to be
> expected when playing games with unsafePerformIO.

Seems you are right. Monomorphic type works, polymorphic doesn't. But it 
probably is not in any way guaranteed to stay like this in future.

> You might get it working by giving iorefset a monomorphic type or by
> specializing it for the type(s) you are using it at.  Don't forget the
> NOINLINE pragma.  I wouldn't do it this way, though.  If you're parsing,
> chances are that your code is monadic anyway.  Put a StateT over the
> parser monad and everything works without black magic.  Even better, if
> you're using parsec you can just put the Map in the user state.

This will create intern-per-parse, which isn't bad and has it's 
advantages, but I wanted to do something global. Anyway it was 
interesting experiment :)

> Udo.


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