I haven't tested this idea, but it occurred to me that this might be a good
place for data families:
data Z = Z
newtype S n = S n
-- Some nastiness to avoid having to see into the future?
type family Pred n :: *
type instance Pred (S n) = n
type instance Pred Z = Z
class MyDataClass ver where
data MyData ver :: *
upgrade :: MyData (Pred ver) -> MyData ver
-- other methods that won't change from version to version
class Less a b
instance Less Z a
instance Less a b => Less a (S b)
convert :: Less a b => MyData a -> MyData b
-- you may need a method in Less to witness the less-ness, but then you'd
iterate your upgrade until you reach the version you want.
-- Then you might want to abstract over this class with an existential, so
it doesn't infect other things:
data MyDataGeneral = forall ver. MyDataClass ver => MyDataGeneral ver
-- now make instances for versions you have, with data instances for your
current version of the structure.
This might not even compile as I just wrote it into my email client, but it
seems like it could work. Any comments?
Dan
On Thu, Dec 16, 2010 at 1:26 PM, Dmitry V'yal
Greetings,
while developing my neural net simulator I stumbled upon a problem.
I have a data type NeuralNet and use Show and Read instances for saving and loading configurations. As time passed, I changed the data type, so the program can no longer load files saved in previous versions.
I want fix it. My current idea looks as follows. I'm going to create a bunch of types NN1, NN2, NN3..NNn for different versions and write converters c12 :: N1 -> N2, c23 :: N2 -> N3 and so on.
But how to organize the whole process of parsing String into NNn so it's easy to change formats? Something based on using a list of parsers [read, c43 . read, c43 . c23 . read, c43, c23 . c12 . read, c43 . c32 . c21 . read]
looks rather verbose and grows quadratically with N.
I'm sure there must be a more elegant way. Any ideas?
Dmitry
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