CouchDB
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1 CouchDB
CouchDB Haskell package is the Haskell interface to the couchDB database software. I appreciate the efforts of Arjun Guha and Brendan Hickey to construct this interface - it is really most convenient to use!
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], which is probably compiled with an slightly earlier version of CouchDB than the current 0.10.1.
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 Multiple functions to store and retrieve notes
Notes have a title, a text and a set of tags. We need functions to store a new note, to update the content of a note and to retrieve all notes with a given word in the title, in the text or as a tag; each of these functions return a list of pairs (doc_id, title) from which the correct one is selected and then retrieved with the doc_id.
I have decided that the notes have IDs produced by couchDB; this assures that changes, even changes in title, continue the same object with a new version.
{-# LANGUAGE DeriveDataTypeable , ScopedTypeVariables #-} module Notes2 (storeNewNote , changeNoteContent , updateNote , deleteNote , retrieveNote , findNoteByTitle , findNoteByTag , findNoteByContent ) where import Database.CouchDB (rev, deleteDoc, CouchView(..), newView, Doc(..), isDocString, queryView, getAndUpdateDoc, DB(..), getDoc, newDoc, db, doc, newNamedDoc, runCouchDB', Rev(..)) -- , couchViewToJSON) import Data.Data (Data, Typeable) import Text.JSON import Text.JSON.Pretty (pp_value) import Text.JSON.Pretty (render) import Database.CouchDB.JSON (jsonObject) import Text.JSON.Generic (toJSON, fromJSON) type Strings = [String] -- basic -- from Henry Laxen's code: type QueryViewResult = (Database.CouchDB.Doc,JSValue) ppJSON = putStrLn . render . pp_value justDoc :: (Data a) => Maybe (Database.CouchDB.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" ----- docid_doc :: (Data a) => QueryViewResult -> (Doc, a) docid_doc (d, x) = (d, stripResult . fromJSON $ x) where stripResult (Ok z) = z stripResult (Error s) = error $ "JSON error " ++ s --------------------- some example data mynotes = db mynotesString mynotesString = "firstnotes1" -- the name of the couchdb n0 = Note "a59" "a1 text vv 45" ["tag1", "tag2"] n1 = Note "a56" "a1 text vv 45" ["tag1", "tag3"] n2 = Note "a56" "updated a1 text vv 45" ["tag1", "tag2", "tag4"] ------------------------------- data Note = Note {title, text :: String, tags :: Strings} deriving (Eq, Ord, Show, Read , Typeable, Data) -- not yet necessary storeNewNote :: DB -> Note -> IO () storeNewNote db note = do let jnote = toJSON note -- the ID is set by couchdb (doc, rev) <- runCouchDB' $ newDoc db jnote putStrLn $ "stored note" ++ show doc ++ " " ++ show rev return () changeNoteContent :: DB -> Doc -> Note -> IO () changeNoteContent db docid newnote = do ret <- runCouchDB' $ getAndUpdateDoc db (docid) (updateNote newnote) let rev = maybe (error "update did not succeed") id ret putStrLn $ "changed " ++ show rev return () updateNote :: Note -> JSValue -> IO JSValue updateNote new old = return . const (toJSON new) $ old deleteNote :: DB -> Doc -> Rev -> IO () deleteNote db id r = do ret <- runCouchDB' $ deleteDoc db (id, r) putStrLn $ "deleted " ++ show ret return () retrieveNote :: DB -> Doc -> IO Note retrieveNote db docid = do ret <- runCouchDB' $ getDoc db docid -- :: Maybe (Doc, Rev, JSValue) -- assumes the note title is the key let n = justDoc ret :: Note putStrLn $ "stored note" ++ show n return n findNoteByTitle :: DB -> String -> IO [(Doc, Note)] findNoteByTitle db tit = do putStrLn $ "search by title using view 'titles' " ++ show tit ret :: [QueryViewResult] <- runCouchDB' $ do queryView db (doc designdoc) (doc "bytitle") [("key", toJSON tit)] putStrLn $ show ret let ls = map docid_doc ret putStrLn $ "result " ++ show ls return ls findNoteByTag :: DB -> String -> IO [(Doc, Note)] findNoteByTag db ta = do putStrLn $ "search by title using view 'titles' " ++ show ta ret :: [QueryViewResult] <- runCouchDB' $ do queryView db (doc designdoc) (doc "bytags") [("key", toJSON ta)] putStrLn $ show ret let ls = map docid_doc ret putStrLn $ "result " ++ show ls return ls findNoteByContent :: DB -> String -> IO [(Doc, Note)] findNoteByContent db word = do putStrLn $ "search by title using view 'titles' " ++ show word ret :: [QueryViewResult] <- runCouchDB' $ do queryView db (doc designdoc) (doc "bywords") [("key", toJSON word)] putStrLn $ show ret let ls = map docid_doc ret putStrLn $ "result " ++ show ls return ls -------------------------------------------------- titleView = "function(doc) { if (doc.title && doc.text && doc.tags) \ \ {emit(doc.title, doc._id); } }" titleView2 = ViewMap "bytitle" titleView tagView = "function(doc) { if (doc.title && doc.text && doc.tags) \ \ { var len=doc.tags.length; \ \ for(var i=0; i<len; i++) { \ \ var value = doc.tags[i]; \ \ emit(value, doc._id); } \ \ } \ \ }" -- tried in temporary views of Futon tagView2 = ViewMap "bytags" tagView wordView = "function (doc) { if (doc.title && doc.text && doc.tags) \ \ { var words = doc.text.split (' '); \ \ var len=words.length; \ \ for(var i=0; i<len; i++) { \ \ var value = words[i]; \ \ emit(value, doc._id); } \ \ } \ \ }" -- use ' for strings inside a haskell string -- escapes are not removed when converting to JSON ? wordView2 = ViewMap "bywords" wordView --- setting views: designdoc = "five" -- "six" -- change for each edit -- cannot update! setViews dbstring = do r <- runCouchDB' $ newView dbstring designdoc [titleView2, tagView2, wordView2] -- inconsistency: here a string, not a DB type ! putStrLn $ "view stored" return () ---- run2 = do storeNewNote mynotes n0 retrieveNote mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6") changeNoteContent mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6") n2 retrieveNote mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6") return () --stored note68d5cfa3622c4586a7a1bfc695e72765 1-dee34e8ccb4ef3ceb9a7dcfb3d7cd20d --stored noteNote {title = "a1234", text = "a1 text vv 45", tags = ["tag1"]} --changed 2-53bfc49b385805020194a59b39fd5ffb --stored noteNote {title = "a56", text = "updated a1 text vv 45", tags = ["tag1","tag2","tag4"]} -- error when duplicate is not appropriate: --stored note"\"*** Exception: src/Database/CouchDB/Unsafe.hs:80:10-63: Irrefutable pattern failed for pattern (Text.JSON.Types.JSObject errorObj) run3 :: IO () = do setViews mynotesString return () run4 :: IO () = do ls :: [(Doc, Note)] <- findNoteByTitle mynotes "a55" putStrLn $ "found by title " ++ show ls return ()
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.
