Database.CouchDB.ViewServer.Reduce

Map Functions

type ReduceSignature a = [Value] -> [Value] -> Bool -> ViewReduce a

The type of your reduce functions as they are stored in CouchDB. The trivial example:

 \keys values rereduce -> return Null

type ViewReduce a = ViewReduceT Parser a

The monad within which a reduce computation takes place. This is a transformation of the Parser monad, although the precise nature and depth of the transformation is an internal detail and subject to change. ViewReduceT is guaranteed to be an instance of the MonadParser class, allowing you to parse JSON structures.

JSON Parsing

JSON parsers lifted into our view monads. This also exports one or two useful symbols from Data.Aeson.Types.

class Monad m => MonadParser m where

Like MonadIO, but for Parser. This allows JSON parsing operations to be lifted into our various view monads.

Methods

liftParser :: Parser a -> m a

Instances

MonadParser Parser

(Monoid w, MonadParser m) => MonadParser (WriterT w m)

parseJSON :: (MonadParser m, FromJSON a) => Value -> m a

Attempts to parse a JSON value into a given type. This is typically used with a type annotation to indicate the target type. If the value can not be parsed into that type, the entire computation will fail.

parseJSONList :: (MonadParser m, FromJSON a) => [Value] -> m [a]

Applies parseJSON to a list of values. This is commonly used with the reduce function arguments.

(.:) :: (MonadParser m, FromJSON a) => Object -> Text -> m a

Parses a required field of an object. If the field is not present, or the value can not be parsed into the target type, the computation will fail.

(.:?) :: (MonadParser m, FromJSON a) => Object -> Text -> m (Maybe a)

Parses an optional field of an object. This will not halt the computation on failure.

(.=) :: ToJSON a => Text -> a -> Pair

Construct a Pair from a key and a value.

object :: [Pair] -> Value

Create a Value from a list of name/value Pairs. If duplicate keys arise, earlier keys and their associated values win.

data Value

A JSON value represented as a Haskell value.

Constructors

Object Object

Array Array

String Text

Number Number

Bool !Bool

Null

Instances

Eq Value

Data Value

Show Value

Typeable Value

IsString Value

ToJSON Value

FromJSON Value

NFData Value

Hashable Value

ViewReduce Monads

logMsg :: String -> ViewReduce ()

Send a log message to the CouchDB server. Note that log messages are only sent if the computation succeeds. If you want to log a message in the event of a failure, look at Alternative.