[Haskell-cafe] A curios monad

[Andrew Coppin]

Ryan Ingram wrote:
> Congratulations, you're halfway to reinventing ST! :)
>   

Except that, AFAIK, ST doesn't provide the "hey you can store anything 
and retrieve it later" trick. ;-)

I did however wonder if there wasn't some way I could use an extra 
phantom type to somehow "tag" which thread a key was generated in... but 
I couldn't figure out how to make it work.

> Here's the "trick" [1]:
>
>   
>> data Storage s x
>> ...
>> data Key s v
>> ...
>>     
>
> Now add the extra "s" parameter to all the functions that use Storage & Key.
>
>   
>> run :: (forall s. Storage s x) -> x
>>     
>
> Now you can't save keys between sessions; the type "s" isn't allowed
> to escape the "forall" on the left of the function arrow!
>   

Ah, I see. It sounds so easy when you already know how... :-)

BTW, does anybody know how rank-N types are different from existential 
types?