Composite.Opaleye.TH

Documentation

deriveOpaleyeEnum :: Name -> String -> (String -> Maybe String) -> Q [Dec]

Derive the various instances required to make a Haskell enumeration map to a PostgreSQL enum type.

In deriveOpaleyeEnum ''HaskellType "sqltype" hsConToSqlValue, ''HaskellType is the sum type (data declaration) to make instances for, "sqltype" is the PostgreSQL type name, and hsConToSqlValue is a function to map names of constructors to SQL values.

The function hsConToSqlValue is of the type String -> Maybe String in order to make using stripPrefix convenient. The function is applied to each constructor name and for Just value that value is used, otherwise for Nothing the constructor name is used.

For example, given the Haskell type:

    data MyEnum = MyFoo | MyBar

And PostgreSQL type:

    CREATE TYPE myenum AS ENUM(foo, bar);

The splice:

    deriveOpaleyeEnum ''MyEnum "myenum" (stripPrefix "my" . map toLower)

Will create PGMyEnum and instances required to use MyEnum / Column MyEnum in Opaleye.

The Haskell generated by this splice for the example is something like:

    data PGMyEnum

    instance FromField MyEnum where
      fromField f mbs = do
        tname <- typename f
        case mbs of
          _ | tname /= "myenum" -> returnError Incompatible f ""
          Just "foo" -> pure MyFoo
          Just "bar" -> pure MyBar
          Just other -> returnError ConversionFailed f ("Unexpected myenum value: " <> unpack other)
          Nothing    -> returnError UnexpectedNull f ""

    instance QueryRunnerColumnDefault PGMyEnum MyEnum where
      queryRunnerColumnDefault = fieldQueryRunnerColumn

    instance Default Constant MyEnum (Column PGMyEnum) where
      def = constantColumnUsing (def :: Constant String (Column PGText)) $  case
        MyFoo -> "foo"
        MyBar -> "bar"