[Yesod] how to write PersistFIeld instance?

Katsutoshi Itoh

3 Feb 2011 3 Feb '11

11:34 a.m.

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?

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Michael Snoyman

3 Feb 3 Feb

12:14 p.m.

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...

Max Cantor

1:40 p.m.

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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Michael Snoyman

1:44 p.m.

That's a good idea. I'm not sure if it will really be more efficient for simple enumeration types, but it would make sense to include some helper code in Persistent. Anyone feel like writing a patch, or at least opening a Github issue so I don't forget? Michael On Thu, Feb 3, 2011 at 8:40 PM, Max Cantor wrote:

...

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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Katsutoshi Itoh
Max Cantor
Michael Snoyman