Re: [web-devel] [Yesod] how to write PersistFIeld instance?

A more general and far more efficient way is to use the Serialize typeclass in the Cereal (http://hackage.haskell.org/package/cereal) instead of read/show for [un]marshalling. You can use makeSerialize (i think) TH in the Data.Derive package to automatically derive instances for Serialize. Finally, the PersistField instance declaration for any instance of Serialize is pretty straight forward: instance PersistField SurveyQuestionList where toPersistValue (SurveyQuestionList l) = PersistByteString $ Cereal.encode l fromPersistValue (PersistByteString b) = fmap SurveyQuestionList $ Cereal.decode b fromPersistValue _ = Left "bad type for survey question list" sqlType _ = SqlBlob isNullable _ = False mc On Feb 4, 2011, at 1:14 AM, Michael Snoyman wrote:

On Thu, Feb 3, 2011 at 6:34 PM, Katsutoshi Itoh wrote:

...

I'd like to introduce serializable data type like this:

data Role = Teacher | Student deriving (Read, Show, Eq, Ord)

share2 mkPersist (mkMigrate "migrateAll") [$persist| User ident String : : role Role Maybe Update : |]

instance PersistField Role where ??? Would you teach me how to write this instance?

Check out the Haskellers codebase[1]: Persistent includes a TH function called derivePersistField[2] which does this automatically for you based on Show/Read instances. I personally find this the best approach to the problem. It's more efficient to serialize to integers (and you could use an Enum instance for that), but it's less future-proof.

Michael

[1] https://github.com/snoyberg/haskellers/blob/master/Model.hs [2] http://hackage.haskell.org/packages/archive/persistent/0.3.1.3/doc/html/Data...

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