Hi Cafe. I am searching for materials on one data type-related problem.
Suppose we have version control system storing values of user-defined algebraic
data type. 'User' (actually, programmer) wants to store his/her data and later
update it by applying patches.
By patch I mean value of some (another) type representing one simplest update
of stored value.
Say, for tree type
> data Tree a = Node (Tree a) (Tree a) | Leaf a
one may write
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE UndecidableInstances #-}
>
> class Storable a where
> type Patch a :: *
> update :: Patch a -> a -> a
>
> data TreeStep = Left | Right -- Like in zippers ?
> type family Location a :: *
> type instance Location (Tree a) = [TreeStep]
>
> instance (Storable a) => Storable (Tree a) where
> type Patch (Tree a) = (Location (Tree a), Patch a)
> update = ... -- Move to specific location and call node's update
Patch here is coded explicitly but I guess there may be some generic way of it
to calculate. Whether any similar work has been carried out?
Will be glad to read comments!
--
Thanks,
Sergey
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