be8a00f · typeable/postgresql-simple

git.iko.soy/typeable/postgresql-simple

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Add FromField.optionalField

be8a00fLeon P Smith10 years ago

src/Database/PostgreSQL/Simple/FromField.hs+15−4

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unifiedsplit

@@ -107,6 +107,7 @@ module Database.PostgreSQL.Simple.FromField

107 , PQ.Format(..)

108 , pgArrayFieldParser

109

110+ , optionalField

110 , fromJSONField

111 , fromJSONField

111 ) where

112 ) where

112

113

@@ -268,9 +269,20 @@ instance FromField () where

268-- compatible with type @a@. Note that the type is not checked if

269-- compatible with type @a@. Note that the type is not checked if

269-- the value is null, although it is inadvisable to rely on this

270-- the value is null, although it is inadvisable to rely on this

270-- behavior.

271-- behavior.

271−instance (FromField a) => FromField (Maybe a) where

272+instance FromField a => FromField (Maybe a) where

273+ fromField = optionalField fromField

274+

275+-- | For dealing with SQL @null@ values outside of the 'FromField' class.

276+-- Alternatively, one could use 'Control.Applicative.optional', but that

277+-- also turns type and conversion errors into 'Nothing', whereas this is

278+-- more specific and turns only @null@ values into 'Nothing'.

279+

280+optionalField :: FieldParser a -> FieldParser (Maybe a)

281+optionalField p f mv =

282+ case mv of

272− fromField _ Nothing = pure Nothing

283+ Nothing -> pure Nothing

273− fromField f bs = Just <$> fromField f bs

284+ Just _ -> Just <$> p f mv

285+{-# INLINE optionalField #-}

274

286

275-- | compatible with any data type, but the value must be null

287-- | compatible with any data type, but the value must be null

276instance FromField Null where

288instance FromField Null where

@@ -567,7 +579,6 @@ fromJSONField f mbBs = do

567 "JSON decoding error: " ++ err

579 "JSON decoding error: " ++ err

568 JSON.Success x -> pure x

580 JSON.Success x -> pure x

569

581

570−

571-- | Compatible with the same set of types as @a@. Note that

582-- | Compatible with the same set of types as @a@. Note that

572-- modifying the 'IORef' does not have any effects outside

583-- modifying the 'IORef' does not have any effects outside

573-- the local process on the local machine.

584-- the local process on the local machine.