couch-hs-0.1.2: A CouchDB view server for Haskell.




This ia a CouchDB view server in and for Haskell. With it, you can define design documents that use Haskell functions to perform map/reduce operations. Database.CouchDB.ViewServer is just a container; see the submodules for API documentation.



This package includes the executable that runs as the CouchDB view server as well as some modules that your map and reduce functions will compile against. This means, for instance, that if CouchDB is running as a system user, this package must be installed globally in order to work.

The executable is named couch-hs. Without any arguments, it will run as a view server, processing lines from stdin until EOF. There are two options that are important to the compilation of your map and reduce functions (couch-hs -h will print a short description of all options).

-x EXT
Adds a language extension to the function interpreters. OverloadedStrings is included by default.
Imports a module into the function interpreter context. You may include a qualified name or leave it unqualified. The default environment is equivalent to the following (the last entry varying for map and reduce functions):
import Prelude
import Data.Maybe
import Data.List as L
import Data.Map as M
improt Data.Text as T
import Data.Aeson.Types as J
import Control.Monad
import Control.Applicative
import Database.CouchDB.ViewServer.[Map|Reduce]

Assuming the package is installed properly, just add it to your CouchDB config file:

haskell = /path/to/couch-hs [options]

Development Modes

In addition to the server mode, couch-hs has some special modes to aid development. CouchDB isn't very good at reporting errors in view functions, so the following modes can be used to make sure your functions compile before installing them into a view. To ensure valid results, be sure to match the couch-hs options with those in CouchDB's config file.

couch-hs [options] -M [CODE|@PATH] ...
Attempt to compile one or more map functions. Each argument can either be a source string or a path to a file prefixed by @. If no arguments are given, one function will be read from stdin. For each map function that is successfully compiled, couch-hs will print OK. If any function fails, the interpreter error(s) will be printed. If there are any failures, couch-hs will exit with a non-zero status.
couch-hs [options] -R [CODE|@PATH] ...
The same as -M, except to compile reduce functions.



Here is a simple summation example to get started. This example assumes documents of the form:

{"name": "Bob", "value": 5}

The map function emits name/value pairs:

\doc -> emitM
  (doc .: "name" :: ViewMap String)
  (doc .: "value" :: ViewMap Integer)

The reduce function adds up all of the values:

\keys values rereduce -> sum <$> parseJSONList values :: ViewReduce Integer

The key things to note here:

  • Map and reduce operations take place in a monadic context. The map and reduce monads are transformers on top of Parser, which is used to parse the decoded JSON into native values. Lifted parsing tools are provided for convenience.
  • Both map and reduce functions will parse JSON values and produce output and log messages. If any JSON parsing operation fails, the entire computation will fail and no results nor log messages will be returned to the server. To handle parse failures, you can use Alternative or .:?.
  • Both map and reduce computations are parameterized in some way. In the case of map functions, it's the emit function; for the reduce functions, it's the return type. In either case, since there is no top-level type annotation, it will be necessary to include annotations at key points in the functions. I find that annotations usually belong at the points where the JSON objects are parsed.

Map Functions

A map function takes a single JSON object as an argument and evaluates to ViewMap (). The map computation may call emit or emitM to returnkey/value pairs for the given document. The emit functions accept any type that can be converted by toJSON, which is a long list. If you want to emit null, pass Null or Nothing (Null is easier, as it doesn't require annotation).

Map functions will generally use .: and .:? to access fields in the object and may need parseJSON to parse embedded values.

If the map computation fails, the result will be equivalent to return ().

type MapSignature = Object -> ViewMap ()Source

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

 \doc -> return ()

Reduce Functions

A reduce function takes three arguments: a list of keys as JSON Values, a list of values as JSON Values, and a Bool for rereduce. The ViewReduce monad may wrap any value that can be converted by toJSON; a type annotation will generally be necessary.

A reduce function will normally use parseJSONList to parse the JSON values into primitive types for processing.

If the reduce computation fails, the result will be equivalent to return Null.

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

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

 \keys values rereduce -> return Null


Here's a larger example that shows off a more practical application. Suppose a set of documents representing shared expenses. We'll include a couple of malformed documents for good measure.

{"date": "2011-06-05", "what": "Dinner", "credits": {"Alice": 80}, "shares": {"Alice": 1, "Bob": 2, "Carol": 1}}
{"date": "2011-06-17", "credits": {"Bob": 75}, "shares": {"Bob": 1, "Doug": 1}}
{"date": "2011-06-08", "what": "Concert", "credits": {"Carol": 150}, "shares": {"Alice": 1, "Carol": 1, "Doug": 1}}
{"date": "2011-05-25", "what": "Bogus", "credits": {"Alice": 50}, "shares": {"Bob": 0}}
{"food": "pizza", "toppings": ["mushrooms", "onions", "sausage"]}

The following map function will calculate the total credit or debt for each person for each valid document. The what field is carried along. The reduce function sums all of the nets to produce the bottom line.

\doc -> let net credits shares = let debts = shareAmounts (sumMap credits) (sumMap shares) shares
                                 in  M.unionWith (+) credits debts

            sumMap = M.fold (+) 0
            shareAmounts totCredit totShares = (\shares -> -(shares / totShares) * totCredit)

        in  do date <- doc .: "date" :: ViewMap T.Text
               what <- doc .:? "what" :: ViewMap (Maybe T.Text) -- Optional field
               credits <- doc .: "credits" :: ViewMap (M.Map T.Text Double)
               shares <- doc .: "shares" :: ViewMap (M.Map T.Text Double)

               guard $ (sumMap shares) > 0  -- Just say no to (/ 0)

               emit date $ object ["net" .= net credits shares, "what" .= what]
\_ values _ -> do objects <- parseJSONList values :: ViewReduce [J.Object]
                  nets <- mapM (.: "net") objects :: ViewReduce [M.Map T.Text Double]
                  return $ L.foldl' (M.unionWith (+)) M.empty nets

Map results:

"2011-06-05": {what: "Dinner", net: {Alice: 60, Bob: -40, Carol: -20}}
"2011-06-08": {what: "Concert", net: {Alice: -50, Carol: 100, Doug: -50}}
"2011-06-17": {what: null, net: {Bob: 37.5, Doug: -37.5}}

Which reduces to:

{Alice: 10, Bob: -2.5, Carol: 80, Doug: -87.5}

API Documentation