PostGIS support and/or custom types tutorial

[tysonzero]

I'm currently getting Opaleye working with PostGIS and it'd be awesome if that was supported out of the box, but even if not it'd be a lot easier with a good tutorial for defining custom sql types with custom serializers (in my case using geos readHex/writeHex functions).

[tysonzero]

This is the code I currently have for some basic PostGIS stuff for those that are curious:

data SqlGeographyPoint

instance Default ToFields (V2 Double) (Field SqlGeographyPoint) where
    def = toToFields $ \(V2 x y) -> let
        geo = Geos.PointGeometry (Geos.Point $ Geos.Coordinate2 x y) (Just 4326)
        in unsafeCast "geography(POINT, 4326)" . toFieldsI . T.decodeUtf8Lenient $ Geos.writeHex geo

instance DefaultFromField SqlGeographyPoint (V2 Double) where
    defaultFromField = fromPGSFieldParser $ \f mv -> do
        nm <- PGS.typename f
        when (nm /= "geography") $ PGS.returnError PGS.Incompatible f ""
        v <- maybe (PGS.returnError PGS.UnexpectedNull f "") pure mv
        case Geos.readHex v of
            Just (Geos.Some (PointGeometry (Geos.Point (Coordinate2 x y)) (Just 4326))) -> do
                pure $ V2 x y
            _ -> PGS.returnError PGS.ConversionFailed f ""

stDistance :: Field SqlGeographyPoint -> Field SqlGeographyPoint -> Field SqlFloat8
stDistance (Column a) (Column b) = Column $ HPQ.FunExpr "ST_Distance" [a, b]

The stDistance function does seem to require importing from Internal modules which is not totally ideal, but it does seem to work at least.