[Haskell-cafe] Replacing [a] with (Set c a) in Monad instance.
Bryan Donlan
bd.haskell at uguu.us
Tue Jan 30 20:06:03 EST 2007
Daniel McAllansmith wrote:
> Hello.
>
> Given:
>
> newtype Dist a = D {unD :: [(a,Int)]}
>
> instance Monad Dist where
> return x = D [(x,1)]
> d >>= f = D [(y,q*p) | (x,p) <- unD d, (y,q) <- unD (f x)]
> fail _ = D []
>
>
> How would one change Dist to wrap an instance of the (Data.Edison.Set c a)
> typeclass so that the Monad instance could be implemented in terms of e.g.
> singleton, unionWith, empty, etc?
I don't know about Data.Edison.Set, but if it's anything like
base/Data.Set, then there's an Ord constraint on the elements, making it
impossible to directly transform into a monad. However, Roberto Zunino
came up with a clever way to bypass this problem with GADTS:
http://article.gmane.org/gmane.comp.lang.haskell.cafe/18118
You may be able to apply this to your situation, using various Edison
collections depending on which typeclasses your monad argument implements.
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