I'd like to store aeson's Value type using acid

Question

I'd like to store aeson Values usig acid-store. I've taken a minimal acid implementation and tried to naively switch the types over to Value. These are my calls to deriveSafeCopy:

$(deriveSafeCopy 0 'base ''Object)
$(deriveSafeCopy 0 'base ''Array)
$(deriveSafeCopy 0 'base ''Number)
$(deriveSafeCopy 0 'base ''Value)
$(deriveSafeCopy 0 'base ''JSONState)
$(deriveSafeCopy 0 'base ''JSONStateStore)

JSONState and JSONStateStore are my own types. I get this error:

Can't derive SafeCopy instance for: (Data.Aeson.Types.Internal.Object,TyConI (TySynD Data.Aeson.Types.Internal.Object [] (AppT (AppT (ConT Data.HashMap.Base.HashMap) (ConT Data.Text.Internal.Text)) (ConT Data.Aeson.Types.Internal.Value))))

Thanks - do you think I should remove the update from the question? I'm not sure about SO etiquette. — schellsan, Apr 24 '13 at 23:16

score 3 · Accepted Answer · answered Apr 23 '13 at 01:06

3

Evidently you've reached a limit of what the deriveSafeCopy Template Haskell function can do for you.

You can solve the issue by providing the instances manually. SafeCopy API contains comprehensive docs on how to do that. For additional examples you can check out how the default instances are declared.

answered Apr 23 '13 at 01:06

Nikita Volkov

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Thank you Nikita, that's what I expected. I think I just needed to hear it from someone. Thanks for a push in the right direction. – schellsan Apr 23 '13 at 01:30
I added my solution above. – schellsan Apr 24 '13 at 22:44

schellsan · Answer 2 · 2014-03-04T06:50:00.433

3

Here is my implementation for those who are still interested:

-- | ACID

$(deriveSafeCopy 0 'base ''JSONStateStore)
$(deriveSafeCopy 0 'base ''JSONState)
$(deriveSafeCopy 0 'base ''Value)
$(deriveSafeCopy 0 'base ''Number)

-- | An instance of SafeCopy for the Array Value.
instance SafeCopy a => SafeCopy (V.Vector a) where
    getCopy = contain $ fmap V.fromList safeGet 
    putCopy = putCopy . V.toList

-- | An instance of SafeCopy for the Object Value.
instance (SafeCopy a, Eq a, Hashable a, SafeCopy b) => SafeCopy (H.HashMap a b) where
    getCopy = contain $ fmap H.fromList safeGet 
    putCopy = contain . safePut . H.toList

edited Mar 04 '14 at 06:50

answered Apr 24 '13 at 23:15

schellsan

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The putCopy definitions are incorrect and will not work as expected. They will compile however, so this is quite dangerous. The correct putCopy implementations should be (contain . safePut . toList) – Reite Feb 27 '14 at 11:02

I'd like to store aeson's Value type using acid

2 Answers2