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1 CouchDB

CouchDB Haskell package is the Haskell interface to the couchDB database software. CouchDB is a document oriented datastorage system (with versions) which is geared towards replication. For more information read Anderson, Lehnardt and Slater's book "CouchDB - The definite guide" ([1]).

Examples how to use the Haskell CouchDB interface are not easy to find on the web. I found only one [2].

I created this wiki page to make available the simple examples I coded to learn to use CouchDB and to report of some of the not-so-obvious traps beginners could fall into.

2 Example: Store and retrieve notes

The simple example I selected is the database backend of a "note" application (e.g. Tomboy or any other of the yellow paste-it notes look-alike). The first function is to store a note, as an example on how to store data in couch:

2.1 Store note

Here the code for storing a note and retrieving it with the doc id:

{-# LANGUAGE DeriveDataTypeable
        , ScopedTypeVariables
module Notes1  where
import Database.CouchDB (getDoc, newDoc, runCouchDB', db, Rev(..), Doc)
import Data.Data (Data, Typeable)
import Text.JSON
import Text.JSON.Pretty (pp_value)
import Text.JSON.Pretty (render)
import Text.JSON.Generic (toJSON, fromJSON)
type Strings = [String]  -- basic
data Note = Note {title, text :: String, tags :: Strings}
    deriving (Eq, Ord, Show, Read , Typeable, Data)  -- not yet necessary
------ copied from henry laxon
ppJSON = putStrLn . render . pp_value
justDoc :: (Data a) => Maybe (Doc, Rev, JSValue) -> a
justDoc (Just (d,r,x)) = stripResult (fromJSON x)
  where stripResult (Ok z) = z
        stripResult (Error s) = error $ "JSON error " ++ s
justDoc Nothing = error "No such Document"
mynotes = db "firstnotes1"
n0 = Note "a59" "a1 text vv 45" ["tag1"]
n1 = Note "a56" "a1 text vv 45" ["tag1"]
n2 = Note "a56" "updated a1 text vv 45" ["tag1"]
n1j = toJSON n1  -- convNote2js n1
runNotes1 = do
            (doc1, rev1) <- runCouchDB' $ newDoc mynotes n1j
            putStrLn $ "stored note" ++ show doc1 ++ "  revision " ++ show rev1
            Just (_,_,jvalue) <- runCouchDB' $ getDoc mynotes doc1
            ppJSON jvalue
            jstuff <- runCouchDB' $ getDoc mynotes doc1
            let d = justDoc jstuff :: Note
            putStrLn $ "found " ++ show d
            return ()
-- the output is:
--stored noteaa45700981408039346f9c8c73f8701f  
--               revision 1-7fa1d1116e6ae0c1ee8d4ce89a701fdf
--{"_id": "aa45700981408039346f9c8c73f8701f",
-- "_rev": "1-7fa1d1116e6ae0c1ee8d4ce89a701fdf", "title": "a56",
-- "text": "a1 text vv 45", "tags": ["tag1"]}
--found Note {title = "a56", text = "a1 text vv 45", tags = ["tag1"]}

2.2 Retrieve note

Notice that the syntax for retrieval has changed since the example code for "converting from MySQL to CouchDB" was written: no preceeding "/_design/" in the name of the view.

3 Coda

I do not guarantee for the correctness of the code (of course). I hope it is useful. I invite others to contribute their examples or more complex codes so we can learn from each other.

I am currently working and interested to hear comments at frank at geoinfo dot tuwien dot ac dot at.