HDBC-1.1.6: Haskell Database Connectivity

MaintainerJohn 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 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 In other words, HDBC has no support for autocommit, which we consider an outdated notion.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 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 ()Source

Commit any pending data to the database.

Required to make any changes take effect.

rollback :: conn -> IO ()Source

Roll back to the state the database was in prior to the last commit or rollback.

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.

data Statement Source




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 Database.HDBC.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.

data SqlError Source

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.



class Show a => SqlType a whereSource

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.

nToSql :: Integral a => a -> SqlValueSource

Converts any Integral type to a SqlValue by using toInteger.

iToSql :: Int -> SqlValueSource

Convenience function for using numeric literals in your program.

data SqlValue Source

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.


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


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" []