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| Database.HDBC.Types | | Portability | portable | | Stability | provisional | | Maintainer | John Goerzen <jgoerzen@complete.org> |
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| Description |
Types for HDBC.
Please note: this module is intended for authors of database driver libraries
only. Authors of applications using HDBC should use HDBC
exclusively.
Written by John Goerzen, jgoerzen@complete.org
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| Synopsis |
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| Documentation |
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| class IConnection conn where |
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we consider an outdated notion.
| | | Methods | | disconnect :: conn -> IO () | 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 Statements
active. In more precise language, the results in such situations are undefined
and vary by database. So don't do it.
| | | commit :: conn -> IO () | Commit any pending data to the database.
Required to make any changes take effect.
| | | rollback :: conn -> IO () | | Roll back to the state the database was in prior to the
last commit or rollback.
| | | run :: conn -> String -> [SqlValue] -> IO Integer | | 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 Statement | 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 conn | 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 -> String | | 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 -> String | | 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 -> String | | 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 -> String | | 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 -> String | | The version of the database server, if available.
| | | dbTransactionSupport :: conn -> Bool | 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] | 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 fetchAllRows'.
All results should be converted to lowercase for you
before you see them.
| | | describeTable :: conn -> String -> IO [(String, SqlColDesc)] | 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 fetchAllRows'.
All results should be converted to lowercase for you
before you see them.
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| |  Instances | |
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| data Statement |
| Constructors | | Statement | | | execute :: ([SqlValue] -> IO Integer) | Execute the prepared statement, passing in the given positional
parameters (that should take the place of the question marks
in the call to prepare).
For non-SELECT queries, the return value is the number of
rows modified, if known. If no rows were modified, you get 0.
If the value is unknown, you get -1. All current HDBC drivers
support this function and should never return -1.
For SELECT queries, you will always get 0.
This function should automatically call finish() to finish the previous
execution, if necessary.
| | executeMany :: ([[SqlValue]] -> IO ()) | Execute the query with many rows.
The return value is the return value from the final row
as if you had called execute on it.
Due to optimizations that are possible due to different
databases and driver designs, this can often be significantly
faster than using execute multiple times since queries
need to be compiled only once.
This is most useful for non-SELECT statements.
| | finish :: (IO ()) | Abort a query in progress -- usually not needed.
| | fetchRow :: (IO (Maybe [SqlValue])) | Fetches one row from the DB. Returns Nothing if there
are no more rows. Will automatically call finish when
the last row is read.
| | getColumnNames :: (IO [String]) | Returns a list of the column names in the result.
For maximum portability, you should not assume that
information is available until after an execute function
has been run.
Information is returned here directly as returned
by the underlying database layer. Note that different
databases have different rules about capitalization
of return values and about representation of names
of columns that are not simple columns. For this reason,
it is suggested that you treat this information for
display purposes only. Failing that, you should convert
to lower (or upper) case, and use AS clauses for
anything other than simple columns.
A simple getColumnNames implementation could simply
apply map fst to the return value of describeResult.
| | originalQuery :: String | The original query that this Statement was prepared
with.
| | describeResult :: (IO [(String, SqlColDesc)]) | Obtain information about the columns in the result set.
Must be run only after execute. 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 fetchAllRows'.
All results should be converted to lowercase for you
before you see them.
Please see caveats under getColumnNames for information
on the column name field here.
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| data SqlError |
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.
| | Constructors | | SqlError | | | seState :: String | | | seNativeError :: Int | | | seErrorMsg :: String | |
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| class Show a => SqlType a where |
Conversions to and from SqlValues 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
See also nToSql, iToSql.
| | | Methods | | | | Instances | |
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| nToSql :: Integral a => a -> SqlValue |
| Converts any Integral type to a SqlValue by using toInteger.
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| iToSql :: Int -> SqlValue |
| Convenience function for using numeric literals in your program.
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| data SqlValue |
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
SqlValues 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.
| | Constructors | | 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
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| | Instances | |
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| data ConnWrapper |
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.
| | Constructors | | forall conn . IConnection conn => ConnWrapper conn | |
| | Instances | |
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| withWConn :: forall b . ConnWrapper -> (forall conn . IConnection conn => conn -> b) -> b |
Unwrap a ConnWrapper and pass the embedded IConnection to
a function. Example:
withWConn wrapped run $ "SELECT * from foo where bar = 1" []
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| Produced by Haddock version 0.8 |
