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  1. FieldAttrMaybe :: FieldAttr

    persistent Database.Persist.Types

    The Maybe keyword goes after the type. This indicates that the column is nullable, and the generated Haskell code will have a Maybe type for it. Example: 

    User
name Text Maybe

  2. lookupMaybe :: Text -> Form -> Either Text (Maybe Text)

    http-api-data Web.FormUrlEncoded

    Lookup an optional value for a key. Fail if there is more than one value. 

    >>> lookupMaybe "name" []
Right Nothing

>>> lookupMaybe "name" [("name", "Oleg")]
Right (Just "Oleg")

>>> lookupMaybe "name" [("name", "Oleg"), ("name", "David")]
Left "Duplicate key \"name\""

  3. parseMaybe :: FromHttpApiData v => Text -> Form -> Either Text (Maybe v)

    http-api-data Web.FormUrlEncoded

    Lookup an optional value for a given key and parse it with parseQueryParam. Fail if there is more than one value for the key. 

    >>> parseMaybe "age" [] :: Either Text (Maybe Word8)
Right Nothing

>>> parseMaybe "age" [("age", "12"), ("age", "25")] :: Either Text (Maybe Word8)
Left "Duplicate key \"age\""

>>> parseMaybe "age" [("age", "seven")] :: Either Text (Maybe Word8)
Left "could not parse: `seven' (input does not start with a digit)"

>>> parseMaybe "age" [("age", "777")] :: Either Text (Maybe Word8)
Left "out of bounds: `777' (should be between 0 and 255)"

>>> parseMaybe "age" [("age", "7")] :: Either Text (Maybe Word8)
Right (Just 7)

  4. parseHeaderMaybe :: FromHttpApiData a => ByteString -> Maybe a

    http-api-data Web.HttpApiData

    Parse HTTP header value in a Maybe. 

    >>> parseHeaderMaybe "hello" :: Maybe Text
Just "hello"

  5. parseQueryParamMaybe :: FromHttpApiData a => Text -> Maybe a

    http-api-data Web.HttpApiData

    Parse query param value in a Maybe. 

    >>> parseQueryParamMaybe "true" :: Maybe Bool
Just True

  6. parseUrlPieceMaybe :: FromHttpApiData a => Text -> Maybe a

    http-api-data Web.HttpApiData

    Parse URL path piece in a Maybe. 

    >>> parseUrlPieceMaybe "12" :: Maybe Int
Just 12

  7. lookupMaybe :: Text -> Form -> Either Text (Maybe Text)

    http-api-data Web.Internal.FormUrlEncoded

    Lookup an optional value for a key. Fail if there is more than one value. 

    >>> lookupMaybe "name" []
Right Nothing

>>> lookupMaybe "name" [("name", "Oleg")]
Right (Just "Oleg")

>>> lookupMaybe "name" [("name", "Oleg"), ("name", "David")]
Left "Duplicate key \"name\""

  8. parseMaybe :: FromHttpApiData v => Text -> Form -> Either Text (Maybe v)

    http-api-data Web.Internal.FormUrlEncoded

    Lookup an optional value for a given key and parse it with parseQueryParam. Fail if there is more than one value for the key. 

    >>> parseMaybe "age" [] :: Either Text (Maybe Word8)
Right Nothing

>>> parseMaybe "age" [("age", "12"), ("age", "25")] :: Either Text (Maybe Word8)
Left "Duplicate key \"age\""

>>> parseMaybe "age" [("age", "seven")] :: Either Text (Maybe Word8)
Left "could not parse: `seven' (input does not start with a digit)"

>>> parseMaybe "age" [("age", "777")] :: Either Text (Maybe Word8)
Left "out of bounds: `777' (should be between 0 and 255)"

>>> parseMaybe "age" [("age", "7")] :: Either Text (Maybe Word8)
Right (Just 7)

  9. parseHeaderMaybe :: FromHttpApiData a => ByteString -> Maybe a

    http-api-data Web.Internal.HttpApiData

    Parse HTTP header value in a Maybe. 

    >>> parseHeaderMaybe "hello" :: Maybe Text
Just "hello"

  10. parseMaybeTextData :: (Text -> Maybe a) -> Text -> Either Text a

    http-api-data Web.Internal.HttpApiData

    Convert Maybe parser into Either Text parser with default error message.

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