Portability | portable |
---|---|
Stability | provisional |
Maintainer | John Goerzen <jgoerzen@complete.org> |
Types for HDBC.
Please note: this module is intended for authors of database driver libraries
only. Authors of applications using HDBC should use Database.HDBC
exclusively.
Written by John Goerzen, jgoerzen@complete.org
- class IConnection conn where
- disconnect :: conn -> IO ()
- commit :: conn -> IO ()
- rollback :: conn -> IO ()
- run :: conn -> String -> [SqlValue] -> IO Integer
- prepare :: conn -> String -> IO Statement
- clone :: conn -> IO conn
- hdbcDriverName :: conn -> String
- hdbcClientVer :: conn -> String
- proxiedClientName :: conn -> String
- proxiedClientVer :: conn -> String
- dbServerVer :: conn -> String
- dbTransactionSupport :: conn -> Bool
- getTables :: conn -> IO [String]
- describeTable :: conn -> String -> IO [(String, SqlColDesc)]
- data Statement = Statement {
- execute :: [SqlValue] -> IO Integer
- executeMany :: [[SqlValue]] -> IO ()
- finish :: IO ()
- fetchRow :: IO (Maybe [SqlValue])
- getColumnNames :: IO [String]
- originalQuery :: String
- describeResult :: IO [(String, SqlColDesc)]
- data SqlError = SqlError {
- seState :: String
- seNativeError :: Int
- seErrorMsg :: String
- class Show a => SqlType a where
- nToSql :: Integral a => a -> SqlValue
- iToSql :: Int -> SqlValue
- data SqlValue
- = SqlString String
- | SqlByteString ByteString
- | SqlWord32 Word32
- | SqlWord64 Word64
- | SqlInt32 Int32
- | SqlInt64 Int64
- | SqlInteger Integer
- | SqlChar Char
- | SqlBool Bool
- | SqlDouble Double
- | SqlRational Rational
- | SqlEpochTime Integer
- | SqlTimeDiff Integer
- | SqlNull
- data ConnWrapper = forall conn . IConnection conn => ConnWrapper conn
- withWConn :: forall b. ConnWrapper -> (forall conn. IConnection conn => conn -> b) -> b
Documentation
class IConnection conn whereSource
Main database handle object.
An IConnection
object is created by specific functions in the module for an
individual database. That is, the connect function -- which creates
this object -- is not standardized through the HDBC interface.
A connection is closed by a call to disconnect
.
A call to commit
is required to make sure that your changes get committed
to the database. In other words, HDBC has no support for autocommit, which
we consider an outdated notion.
disconnect :: conn -> IO ()Source
Disconnect from the remote database.
You do not need to explicitly close an IConnection object, but you may do so if
you so desire. If you don't, the object will disconnect from the database
in a sane way when it is garbage-collected. However, a disconnection may
raise an error, so you are encouraged to explicitly call disconnect
. Also,
garbage collection may not run when the program terminates, and some databases
really like an explicit disconnect.
So, bottom line is, you're best off calling disconnect
directly, but the
world won't end if you forget.
This function discards any data not committed already. Database driver
implementators should explicitly call rollback
if their databases don't
do this automatically on disconnect.
Bad Things (TM) could happen if you call this while you have Statement
s
active. In more precise language, the results in such situations are undefined
and vary by database. So don't do it.
Commit any pending data to the database.
Required to make any changes take effect.
rollback :: conn -> IO ()Source
run :: conn -> String -> [SqlValue] -> IO IntegerSource
Execute a single SQL query. Returns the number
of rows modified (see execute
for details).
The second parameter is a list
of replacement values, if any.
prepare :: conn -> String -> IO StatementSource
Prepares a statement for execution.
Question marks in the statement will be replaced by
positional parameters in a later call to execute
.
Please note that, depending on the database
and the driver, errors in your SQL may be raised
either here or by execute
. Make sure you
handle exceptions both places if necessary.
clone :: conn -> IO connSource
Create a new Connection
object, pointed at the same
server as this object is. This will generally establish
a separate physical connection.
When you wish to establish multiple connections to a single server, the correct way to do so is to establish the first connection with the driver-specific connection function, and then clone it for each additional connection.
This can be important when a database doesn't provide much thread support itself, and the HDBC driver module must serialize access to a particular database.
This can also be a handy utility function whenever you need a separate connection to whatever database you are connected to already.
hdbcDriverName :: conn -> StringSource
The name of the HDBC driver module for this connection. Ideally would be the same as the database name portion of the Cabal package name. For instance, "sqlite3" or "odbc". This is the layer that is bound most tightly to HDBC.
hdbcClientVer :: conn -> StringSource
The version of the C (or whatever) client library that the HDBC driver module is bound to. The meaning of this is driver-specific. For an ODBC or similar proxying driver, this should be the version of the ODBC library, not the eventual DB client driver.
proxiedClientName :: conn -> StringSource
In the case of a system such as ODBC, the name of
the database client/server in use, if available.
For others,
identical to hdbcDriverName
.
proxiedClientVer :: conn -> StringSource
In the case of a system such as ODBC, the version of
the database client in use, if available. For others,
identical to hdbcClientVer
. This is the next layer
out past the HDBC driver.
dbServerVer :: conn -> StringSource
The version of the database server, if available.
dbTransactionSupport :: conn -> BoolSource
Whether or not the current database supports transactions.
If False, then commit
and rollback
should be expected
to raise errors.
MySQL is the only commonly-used database that is known to not support transactions entirely. Please see the MySQL notes in the ODBC driver for more information.
getTables :: conn -> IO [String]Source
The names of all tables accessible by the current connection, excluding special meta-tables (system tables).
You should expect this to be returned in the same manner
as a result from Database.HDBC.fetchAllRows'
.
All results should be converted to lowercase for you before you see them.
describeTable :: conn -> String -> IO [(String, SqlColDesc)]Source
Obtain information about the columns in a specific table. The String in the result set is the column name.
You should expect this to be returned in the same manner
as a result from Database.HDBC.fetchAllRows'
.
All results should be converted to lowercase for you before you see them.
Statement | |
|
The main HDBC exception object. As much information as possible is passed from the database through to the application through this object.
Errors generated in the Haskell layer will have seNativeError set to -1.
SqlError | |
|
class Show a => SqlType a whereSource
Conversions to and from SqlValue
s and standard Haskell types.
Conversions are powerful; for instance, you can call fromSql
on a SqlInt32
and get a String or a Double out of it. This class attempts to Do
The Right Thing whenever possible, and will raise an error when asked to
do something incorrect. In particular, when converting to any type
except a Maybe, SqlNull
as the input will cause an error to be raised.
Here are some notes about conversion:
- Fractions of a second are not preserved on time values
nToSql :: Integral a => a -> SqlValueSource
Converts any Integral type to a SqlValue
by using toInteger.
The main type for expressing Haskell values to SQL databases.
This type is used to marshall Haskell data to and from database APIs. HDBC driver interfaces will do their best to use the most accurate and efficient way to send a particular value to the database server.
Values read back from the server are put in the most appropriate SqlValue
type. fromSql
can then be used to convert them into whatever type
is needed locally in Haskell.
Most people will use toSql
and fromSql
instead of manipulating
SqlValue
s directly.
The default representation of time values is an integer number of seconds. Databases such as PostgreSQL with builtin timestamp types can will see automatic conversion between these Haskell types to local types. Other databases can just use an int or a string.
This behavior also exists for other types. For instance, many databases don't have a Rational type, so they'll just use Haskell's show function and store a Rational as a string.
Two SqlValues are considered to be equal if one of these hold (first one that is true holds; if none are true, they are not equal): * Both are NULL * Both represent the same type and the encapsulated values are equal * The values of each, when converted to a string, are equal.
SqlString String | |
SqlByteString ByteString | |
SqlWord32 Word32 | |
SqlWord64 Word64 | |
SqlInt32 Int32 | |
SqlInt64 Int64 | |
SqlInteger Integer | |
SqlChar Char | |
SqlBool Bool | |
SqlDouble Double | |
SqlRational Rational | |
SqlEpochTime Integer | Representation of ClockTime or CalendarTime |
SqlTimeDiff Integer | Representation of TimeDiff |
SqlNull | NULL in SQL or Nothing in Haskell |
data ConnWrapper Source
Sometimes, it is annoying to use typeclasses with Haskell's type system. In those situations, you can use a ConnWrapper. You can create one with:
let wrapped = ConnWrapper iconn
You can then use this directly, since a ConnWrapper is also an
IConnection
. However, you will not be able to use private database
functions on it.
Or, you can use withWConn
.
forall conn . IConnection conn => ConnWrapper conn |
withWConn :: forall b. ConnWrapper -> (forall conn. IConnection conn => conn -> b) -> bSource
Unwrap a ConnWrapper
and pass the embedded IConnection
to
a function. Example:
withWConn wrapped run $ "SELECT * from foo where bar = 1" []