Snap.Snaplet.PostgresqlSimple

This snaplet makes it simple to use a PostgreSQL database from your Snap application and is based on the excellent postgresql-simple library (http://hackage.haskell.org/package/postgresql-simple) by Leon Smith (adapted from Bryan O'Sullivan's mysql-simple). Now, adding a database to your web app takes just two simple steps.

First, include this snaplet in your application's state.

data App = App
    { ... -- Other state needed in your app
    , _db :: Snaplet Postgres
    }

Next, call the pgsInit from your application's initializer.

appInit = makeSnaplet ... $ do
    ...
    d <- nestSnaplet "db" db pgsInit
    return $ App ... d

Now you can use any of the postgresql-simple wrapper functions defined in this module anywhere in your application handlers. For instance:

postHandler :: Handler App App ()
postHandler = do
    posts <- with db $ query_ "select * from blog_post"
    ...

Optionally, if you find yourself doing many database queries, you can eliminate some of the boilerplate by defining a HasPostgres instance for your application.

instance HasPostgres (Handler b App) where
  getPostgresState = with db get
  setLocalPostgresState s = local (set (db . snapletValue) s)

With this code, our postHandler example no longer requires the with function:

postHandler :: Handler App App ()
postHandler = do
    posts <- query_ "select * from blog_post"
    ...

If you have code that runs multiple queries but you want to make sure that you only use one database connection then you can use the withPG function, like so:

postHandler :: Handler App App ()
postHandler = withPG $ do
    posts <- query_ "select * from blog_post"
    links <- query_ "select * from links"
    ...

The first time you run an application with the postgresql-simple snaplet, a configuration file devel.cfg is created in the snaplets/postgresql-simple directory underneath your project root. It specifies how to connect to your PostgreSQL server and what user, password, and database to use. Edit this file and modify the values appropriately and you'll be off and running.

If you want to have out-of-the-box authentication, look at the documentation for the Snap.Snaplet.Auth.Backends.PostgresqlSimple module.

The Snaplet

data Postgres

The state for the postgresql-simple snaplet. To use it in your app include this in your application state and use pgsInit to initialize it.

Constructors

PostgresPool (Pool Connection)

PostgresConn Connection

Instances

MonadCatchIO m => HasPostgres (ReaderT (Snaplet Postgres) m)

A convenience instance to make it easier to use this snaplet in the Initializer monad like this:

d <- nestSnaplet "db" db pgsInit
count <- liftIO $ runReaderT (execute "INSERT ..." params) d

MonadCatchIO m => HasPostgres (ReaderT Postgres m)

A convenience instance to make it easier to use functions written for this snaplet in non-snaplet contexts.

HasPostgres (Handler b Postgres)

Default instance

class MonadCatchIO m => HasPostgres m where

Instantiate this typeclass on 'Handler b YourAppState' so this snaplet can find the connection source. If you need to have multiple instances of the postgres snaplet in your application, then don't provide this instance and leverage the default instance by using "with dbLens" in front of calls to snaplet-postgresql-simple functions.

Methods

getPostgresState :: m Postgres

setLocalPostgresState :: Postgres -> m a -> m a

Instances

MonadCatchIO m => HasPostgres (ReaderT (Snaplet Postgres) m)

A convenience instance to make it easier to use this snaplet in the Initializer monad like this:

d <- nestSnaplet "db" db pgsInit
count <- liftIO $ runReaderT (execute "INSERT ..." params) d

MonadCatchIO m => HasPostgres (ReaderT Postgres m)

A convenience instance to make it easier to use functions written for this snaplet in non-snaplet contexts.

HasPostgres (Handler b Postgres)

Default instance

data PGSConfig

Data type holding all the snaplet's config information.

Constructors

PGSConfig

Fields

pgsConnStr :: ByteString

A libpq connection string.

pgsNumStripes :: Int

The number of distinct sub-pools to maintain. The smallest acceptable value is 1.

pgsIdleTime :: Double

Amount of time for which an unused resource is kept open. The smallest acceptable value is 0.5 seconds.

pgsResources :: Int

Maximum number of resources to keep open per stripe. The smallest acceptable value is 1.

pgsDefaultConfig

Arguments

:: ByteString

A connection string such as "host=localhost port=5432 dbname=mydb"

-> PGSConfig

Returns a config object with default values and the specified connection string.

mkPGSConfig :: MonadIO m => Config -> m PGSConfig

Builds a PGSConfig object from a configurator Config object. This function uses getConnectionString to construct the connection string. The rest of the PGSConfig fields are obtained from "numStripes", "idleTime", and "maxResourcesPerStripe".

pgsInit :: SnapletInit b Postgres

Initialize the snaplet

pgsInit' :: PGSConfig -> SnapletInit b Postgres

Initialize the snaplet using a specific configuration.

getConnectionString :: Config -> IO ByteString

Produce a connection string from a config

withPG :: HasPostgres m => m b -> m b

Function that reserves a single connection for the duration of the given action. Nested calls to withPG will only reserve one connection. For example, the following code calls withPG twice in a nested way yet only results in a single connection being reserved:

myHandler = withPG $ do
   queryTheDatabase
   commonDatabaseMethod

commonDatabaseMethod = withPG $ do
   moreDatabaseActions
   evenMoreDatabaseActions

This is useful in a practical setting because you may often find yourself in a situation where you have common code (that requires a database connection) that you wish to call from other blocks of code that may require a database connection and you still want to make sure that you are only using one connection through all of your nested methods.

liftPG :: HasPostgres m => (Connection -> IO b) -> m b

Convenience function for executing a function that needs a database connection.

Wrappers and re-exports

query :: (HasPostgres m, ToRow q, FromRow r) => Query -> q -> m [r]

See query

query_ :: (HasPostgres m, FromRow r) => Query -> m [r]

See query_

fold :: (HasPostgres m, FromRow row, ToRow params) => Query -> params -> b -> (b -> row -> IO b) -> m b

foldWithOptions :: (HasPostgres m, FromRow row, ToRow params, MonadCatchIO m) => FoldOptions -> Query -> params -> b -> (b -> row -> IO b) -> m b

fold_ :: (HasPostgres m, FromRow row, MonadCatchIO m) => Query -> b -> (b -> row -> IO b) -> m b

foldWithOptions_ :: (HasPostgres m, FromRow row) => FoldOptions -> Query -> b -> (b -> row -> IO b) -> m b

forEach :: (HasPostgres m, FromRow r, ToRow q) => Query -> q -> (r -> IO ()) -> m ()

forEach_ :: (HasPostgres m, FromRow r) => Query -> (r -> IO ()) -> m ()

execute :: (HasPostgres m, ToRow q) => Query -> q -> m Int64

execute_ :: HasPostgres m => Query -> m Int64

executeMany :: (HasPostgres m, ToRow q) => Query -> [q] -> m Int64

returning :: (HasPostgres m, ToRow q, FromRow r) => Query -> [q] -> m [r]

See returning

withTransaction :: HasPostgres m => m a -> m a

withTransactionLevel :: HasPostgres m => IsolationLevel -> m a -> m a

withTransactionMode :: HasPostgres m => TransactionMode -> m a -> m a

formatMany :: (ToRow q, HasPostgres m) => Query -> [q] -> m ByteString

formatQuery :: (ToRow q, HasPostgres m) => Query -> q -> m ByteString

data Query :: *

A query string. This type is intended to make it difficult to construct a SQL query by concatenating string fragments, as that is an extremely common way to accidentally introduce SQL injection vulnerabilities into an application.

This type is an instance of IsString, so the easiest way to construct a query is to enable the OverloadedStrings language extension and then simply write the query in double quotes.

{-# LANGUAGE OverloadedStrings #-}

import Database.PostgreSQL.Simple

q :: Query
q = "select ?"

The underlying type is a ByteString, and literal Haskell strings that contain Unicode characters will be correctly transformed to UTF-8.

Instances

Eq Query

Ord Query

Read Query

Show Query

IsString Query

Monoid Query

Typeable * Query

newtype In a :: * -> *

Wrap a list of values for use in an IN clause. Replaces a single "?" character with a parenthesized list of rendered values.

Example:

query c "select * from whatever where id in ?" (Only (In [3,4,5]))

Constructors

In a

Instances

Functor In

Eq a => Eq (In a)

Ord a => Ord (In a)

Read a => Read (In a)

Show a => Show (In a)

ToField a => ToField (In [a])

Typeable (* -> *) In

newtype Binary a :: * -> *

Wrap binary data for use as a bytea value.

Constructors

Binary

Fields

fromBinary :: a

Instances

Functor Binary

Eq a => Eq (Binary a)

Ord a => Ord (Binary a)

Read a => Read (Binary a)

Show a => Show (Binary a)

FromField (Binary ByteString)

bytea

FromField (Binary ByteString)

bytea

ToField (Binary ByteString)

ToField (Binary ByteString)

Typeable (* -> *) Binary

newtype Only a :: * -> *

A single-value "collection".

This is useful if you need to supply a single parameter to a SQL query, or extract a single column from a SQL result.

Parameter example:

query c "select x from scores where x > ?" (Only (42::Int))

Result example:

xs <- query_ c "select id from users"
forM_ xs $ \(Only id) -> {- ... -}

Constructors

Only

Fields

fromOnly :: a

Instances

Functor Only

Eq a => Eq (Only a)

Ord a => Ord (Only a)

Read a => Read (Only a)

Show a => Show (Only a)

FromField a => FromRow (Maybe (Only a))

FromField a => FromRow (Only a)

ToField a => ToRow (Only a)

Typeable (* -> *) Only

data SqlError :: *

Constructors

SqlError

Fields

sqlState :: ByteString

sqlExecStatus :: ExecStatus

sqlErrorMsg :: ByteString

sqlErrorDetail :: ByteString

sqlErrorHint :: ByteString

Instances

Show SqlError

Exception SqlError

Typeable * SqlError

data FormatError :: *

Exception thrown if a Query could not be formatted correctly. This may occur if the number of '?' characters in the query string does not match the number of parameters provided.

Constructors

FormatError

Fields

fmtMessage :: String

fmtQuery :: Query

fmtParams :: [ByteString]

Instances

Eq FormatError

Show FormatError

Exception FormatError

Typeable * FormatError

data QueryError :: *

Exception thrown if query is used to perform an INSERT-like operation, or execute is used to perform a SELECT-like operation.

Constructors

QueryError

Fields

qeMessage :: String

qeQuery :: Query

Instances

Eq QueryError

Show QueryError

Exception QueryError

Typeable * QueryError

data ResultError :: *

Exception thrown if conversion from a SQL value to a Haskell value fails.

Constructors

Incompatible

The SQL and Haskell types are not compatible.

Fields

errSQLType :: String

errSQLTableOid :: Maybe Oid

errSQLField :: String

errHaskellType :: String

errMessage :: String

UnexpectedNull

A SQL NULL was encountered when the Haskell type did not permit it.

Fields

errSQLType :: String

errSQLTableOid :: Maybe Oid

errSQLField :: String

errHaskellType :: String

errMessage :: String

ConversionFailed

The SQL value could not be parsed, or could not be represented as a valid Haskell value, or an unexpected low-level error occurred (e.g. mismatch between metadata and actual data in a row).

Fields

errSQLType :: String

errSQLTableOid :: Maybe Oid

errSQLField :: String

errHaskellType :: String

errMessage :: String

Instances

Eq ResultError

Show ResultError

Exception ResultError

Typeable * ResultError

data TransactionMode :: *

Constructors

TransactionMode

Fields

isolationLevel :: !IsolationLevel

readWriteMode :: !ReadWriteMode

Instances

Eq TransactionMode

Show TransactionMode

data IsolationLevel :: *

Of the four isolation levels defined by the SQL standard, these are the three levels distinguished by PostgreSQL as of version 9.0. See http://www.postgresql.org/docs/9.1/static/transaction-iso.html for more information. Note that prior to PostgreSQL 9.0, RepeatableRead was equivalent to Serializable.

Constructors

DefaultIsolationLevel

the isolation level will be taken from PostgreSQL's per-connection default_transaction_isolation variable, which is initialized according to the server's config. The default configuration is ReadCommitted.

ReadCommitted

RepeatableRead

Serializable

Instances

Bounded IsolationLevel

Enum IsolationLevel

Eq IsolationLevel

Ord IsolationLevel

Show IsolationLevel

data ReadWriteMode :: *

Constructors

DefaultReadWriteMode

the read-write mode will be taken from PostgreSQL's per-connection default_transaction_read_only variable, which is initialized according to the server's config. The default configuration is ReadWrite.

ReadWrite

ReadOnly

Instances

Bounded ReadWriteMode

Enum ReadWriteMode

Eq ReadWriteMode

Ord ReadWriteMode

Show ReadWriteMode

begin :: Connection -> IO ()

Begin a transaction.

beginLevel :: IsolationLevel -> Connection -> IO ()

Begin a transaction with a given isolation level

beginMode :: TransactionMode -> Connection -> IO ()

Begin a transaction with a given transaction mode

rollback :: Connection -> IO ()

Rollback a transaction.

commit :: Connection -> IO ()

Commit a transaction.

data h :. t :: * -> * -> * infixr 3

A composite type to parse your custom data structures without having to define dummy newtype wrappers every time.

instance FromRow MyData where ...

instance FromRow MyData2 where ...

then I can do the following for free:

res <- query' c "..."
forM res $ \(MyData{..} :. MyData2{..}) -> do
  ....

Constructors

h :. t infixr 3

Instances

(Eq h, Eq t) => Eq ((:.) h t)

(Ord h, Ord t) => Ord ((:.) h t)

(Read h, Read t) => Read ((:.) h t)

(Show h, Show t) => Show ((:.) h t)

(FromRow a, FromRow b) => FromRow ((:.) a b)

(ToRow a, ToRow b) => ToRow ((:.) a b)

Typeable (* -> * -> *) (:.)

class ToRow a where

A collection type that can be turned into a list of rendering Actions.

Instances should use the toField method of the ToField class to perform conversion of each element of the collection.

Methods

toRow :: a -> [Action]

ToField a collection of values.

Instances

ToRow ()

ToField a => ToRow [a]

ToField a => ToRow (Only a)

(ToField a, ToField b) => ToRow (a, b)

(ToRow a, ToRow b) => ToRow ((:.) a b)

(ToField a, ToField b, ToField c) => ToRow (a, b, c)

(ToField a, ToField b, ToField c, ToField d) => ToRow (a, b, c, d)

(ToField a, ToField b, ToField c, ToField d, ToField e) => ToRow (a, b, c, d, e)

(ToField a, ToField b, ToField c, ToField d, ToField e, ToField f) => ToRow (a, b, c, d, e, f)

(ToField a, ToField b, ToField c, ToField d, ToField e, ToField f, ToField g) => ToRow (a, b, c, d, e, f, g)

(ToField a, ToField b, ToField c, ToField d, ToField e, ToField f, ToField g, ToField h) => ToRow (a, b, c, d, e, f, g, h)

(ToField a, ToField b, ToField c, ToField d, ToField e, ToField f, ToField g, ToField h, ToField i) => ToRow (a, b, c, d, e, f, g, h, i)

(ToField a, ToField b, ToField c, ToField d, ToField e, ToField f, ToField g, ToField h, ToField i, ToField j) => ToRow (a, b, c, d, e, f, g, h, i, j)

class FromRow a where

A collection type that can be converted from a sequence of fields. Instances are provided for tuples up to 10 elements and lists of any length.

Note that instances can be defined outside of postgresql-simple, which is often useful. For example, here's an instance for a user-defined pair:

@data User = User { name :: String, fileQuota :: Int }

instance FromRow User where fromRow = User <$> field <*> field @

The number of calls to field must match the number of fields returned in a single row of the query result. Otherwise, a ConversionFailed exception will be thrown.

Note that field evaluates it's result to WHNF, so the caveats listed in mysql-simple and very early versions of postgresql-simple no longer apply. Instead, look at the caveats associated with user-defined implementations of fromField.

Methods

fromRow :: RowParser a

Instances

FromRow AuthUser

FromField a => FromRow [a]

FromField a => FromRow (Maybe [a])

(FromField a, FromField b) => FromRow (Maybe (a, b))

(FromField a, FromField b, FromField c) => FromRow (Maybe (a, b, c))

(FromField a, FromField b, FromField c, FromField d) => FromRow (Maybe (a, b, c, d))

(FromField a, FromField b, FromField c, FromField d, FromField e) => FromRow (Maybe (a, b, c, d, e))

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f) => FromRow (Maybe (a, b, c, d, e, f))

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g) => FromRow (Maybe (a, b, c, d, e, f, g))

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h) => FromRow (Maybe (a, b, c, d, e, f, g, h))

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i) => FromRow (Maybe (a, b, c, d, e, f, g, h, i))

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i, FromField j) => FromRow (Maybe (a, b, c, d, e, f, g, h, i, j))

FromField a => FromRow (Maybe (Vector a))

FromField a => FromRow (Maybe (Only a))

FromField a => FromRow (Vector a)

FromField a => FromRow (Only a)

(FromField a, FromField b) => FromRow (a, b)

(FromRow a, FromRow b) => FromRow ((:.) a b)

(FromField a, FromField b, FromField c) => FromRow (a, b, c)

(FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d)

(FromField a, FromField b, FromField c, FromField d, FromField e) => FromRow (a, b, c, d, e)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f) => FromRow (a, b, c, d, e, f)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g) => FromRow (a, b, c, d, e, f, g)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h) => FromRow (a, b, c, d, e, f, g, h)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i) => FromRow (a, b, c, d, e, f, g, h, i)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i, FromField j) => FromRow (a, b, c, d, e, f, g, h, i, j)

defaultConnectInfo :: ConnectInfo

Default information for setting up a connection.

Defaults are as follows:

• Server on localhost
• Port on 5432
• User postgres
• No password
• Database postgres

Use as in the following example:

connect defaultConnectInfo { connectHost = "db.example.com" }

defaultTransactionMode :: TransactionMode

defaultIsolationLevel :: IsolationLevel

defaultReadWriteMode :: ReadWriteMode

field :: FromField a => RowParser a