[Haskell-cafe] A curios monad
andrewcoppin at btinternet.com
Thu Dec 11 13:48:29 EST 2008
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" :
>> 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
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