|
Database.HDBC | Portability | portable | Stability | provisional | Maintainer | John Goerzen <jgoerzen@complete.org> |
|
|
|
|
|
Description |
Welcome to HDBC, the Haskell Database Connectivity library.
Written by John Goerzen, jgoerzen@complete.org
|
|
Synopsis |
|
|
|
|
Introduction
|
|
Welcome to HDBC, Haskell Database Connectivity.
HDBC provides an abstraction layer between Haskell programs and SQL
relational databases. This lets you write database code once, in
Haskell, and have it work with any number of backend SQL databases
(MySQL, Oracle, PostgreSQL, ODBC-compliant databases, etc.)
HDBC is modeled loosely on Perl's DBI interface
http://search.cpan.org/~timb/DBI/DBI.pm, though it has also
been influenced by Python's DB-API v2, JDBC in Java, and HSQL in
Haskell.
HDBC is a from-scratch effort. It is not a reimplementation of HSQL,
though its purpose is the same.
|
|
Features
|
|
Features of HDBC include:
- Ability to use replacable parameters to let one query be
executed multiple times (eliminates the need for an escape
function)
- Ability to access returned rows by column number
- Ability to read data from the SQL server on-demand rather than
reading the entire result set up front
- HUnit testsuite for each backend driver
- Well-defined standard API and easy backend driver implementation
- Lazy reading of the entire result set (think hGetContents, but
for the results of SELECT) (see sFetchAllRows)
- Support for translation between Haskell and SQL types
- Support for querying database server properties
- Add-on package (hdbc-missingh) to integrate with MissingH,
providing a database backend for AnyDBM.
- Support for querying metadata such as column names.
- Support for querying additional metadata (column types, etc.)
|
|
Available Drivers
|
|
Here is a list of known drivers as of January 26, 2009:
- Sqlite v3
- Available from http://software.complete.org/hdbc-sqlite3. Or, to
participate in development, use
git clone git://git.complete.org/hdbc-sqlite3
- PostgreSQL
- Available from http://software.complete.org/hdbc-postgresql. Or, to
participate in development, use
git clone git://git.complete.org/hdbc-postgresql
- ODBC
- Available from http://software.complete.org/hdbc-odbc. Or, to
partitipace in development, use
git clone git://git.complete.org/hdbc-odbc
- MySQL
- MySQL users have two choices: the first is the ODBC driver, which works
and has been tested against MySQL on both Linux/Unix and Windows platforms. There is
also an alpha-quality native MySQL driver available for download at
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql with a homepage
at http://www.maubi.net/~waterson/hacks/hdbc-mysql.html.
In addition, there is one integration package: hdbc-anydbm. This
integrates with the AnyDBM library http://software.complete.org/anydbm.
It lets any HDBC database act as a backend for the
AnyDBM interface. Available from http://software.complete.org/hdbc-anydbm. Or,
to participate in development, use
darcs get --partial http://darcs.complete.org/hdbc-anydbm
The latest version of HDBC itself is available from
http://software.complete.org/hdbc. Or, to participate in development, use
git clone git://git.complete.org/hdbc.
|
|
Typing of transfer data
|
|
|
SqlValue is he main type for expressing Haskell values to SQL databases.
INTRODUCTION TO SQLVALUE
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 constructed with the most appropriate SqlValue
constructor. fromSql or safeFromSql
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.
EASY CONVERSIONS BETWEEN 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.
Conversions are implemented in terms of the Data.Convertible module, part of the
convertible package. You can refer to its documentation, and import that module,
if you wish to parse the Left result from safeFromSql yourself, or write your
own conversion instances.
Here are some notes about conversion:
- Fractions of a second are not preserved on time values
- There is no safeToSql because toSql never fails.
See also toSql, safeFromSql, fromSql, nToSql, iToSql, posixToSql.
ERROR CONDITIONS
There may sometimes be an error during conversion. For instance, if you have a
SqlString and are attempting to convert it to an Integer, but it doesn't parse as
an Integer, you will get an error. This will be indicated as an exception if using
fromSql, or a Left result if using safeFromSql.
SPECIAL NOTE ON POSIXTIME
Note that a NominalDiffTime or POSIXTime is converted to SqlDiffTime by
toSql. HDBC cannot differentiate between NominalDiffTime and POSIXTime
since they are the same underlying type. You must construct SqlPOSIXTime
manually or via posixToSql, or use SqlUTCTime.
DETAILS ON SQL TYPES
HDBC database backends are expected to marshal date and time data back and
forth using the appropriate representation for the underlying database engine.
Databases such as PostgreSQL with builtin date and time types should see automatic
conversion between these Haskell types to database types. Other databases will be
presented with an integer or a string. Care should be taken to use the same type on
the Haskell side as you use on the database side. For instance, if your database
type lacks timezone information, you ought not to use ZonedTime, but
instead LocalTime or UTCTime. Database type systems are not always as rich
as Haskell. For instance, for data stored in a TIMESTAMP
WITHOUT TIME ZONE column, HDBC may not be able to tell if it is intended
as UTCTime or LocalTime data, and will happily convert it to both,
upon your request. It is
your responsibility to ensure that you treat timezone issues with due care.
This behavior also exists for other types. For instance, many databases do not
have a Rational type, so they will just use the show function and
store a Rational as a string.
The conversion between Haskell types and database types is complex,
and generic code in HDBC or its backends cannot possibly accomodate
every possible situation. In some cases, you may be best served by converting your
Haskell type to a String, and passing that to the database.
UNICODE AND BYTESTRINGS
Beginning with HDBC v2.0, interactions with a database are presumed to occur in UTF-8.
To accomplish this, whenever a ByteString must be converted to or from a String,
the ByteString is assumed to be in UTF-8 encoding, and will be decoded or encoded
as appropriate. Database drivers will generally present text or string data they have
received from the database as a SqlValue holding a ByteString, which fromSql will
automatically convert to a String, and thus automatically decode UTF-8, when
you need it. In the other direction, database drivers will generally convert
a SqlString to a ByteString in UTF-8 encoding before passing it to the
database engine.
If you are handling some sort of binary data that is not in UTF-8, you can of course
work with the ByteString directly, which will bypass any conversion.
Due to lack of support by database engines, lazy ByteStrings are not passed to database
drivers. When you use toSql on a lazy ByteString, it will be converted to a strict
ByteString for storage. Similarly, fromSql will convert a strict ByteString to
a lazy ByteString if you demand it.
EQUALITY OF SQLVALUE
Two SqlValues are considered to be equal if one of these hold. The
first comparison that can be made is controlling; if none of these
comparisons can be made, then they are not equal:
- Both are NULL
- Both represent the same type and the encapsulated values are considered equal
by applying (==) to them
- The values of each, when converted to a string, are equal.
STRING VERSIONS OF TIMES
Default string representations are given as comments below where such are non-obvious.
These are used for fromSql when a String is desired. They are also defaults for
representing data to SQL backends, though individual backends may override them
when a different format is demanded by the underlying database. Date and time formats
use ISO8601 date format, with HH:MM:SS added for time, and -HHMM added for timezone
offsets.
DEPRECATED CONSTRUCTORS
SqlEpochTime and SqlTimeDiff are no longer created automatically by any
toSql or fromSql functions or database backends. They may still be manually
constructed, but are
expected to be removed in a future version. Although these two constructures will
be removed, support for marshalling to and from the old System.Time data will be
maintained as long as System.Time is, simply using the newer data types for conversion.
| Constructors | SqlString String | | SqlByteString ByteString | | SqlWord32 Word32 | | SqlWord64 Word64 | | SqlInt32 Int32 | | SqlInt64 Int64 | | SqlInteger Integer | | SqlChar Char | | SqlBool Bool | | SqlDouble Double | | SqlRational Rational | | SqlLocalDate Day | Local YYYY-MM-DD (no timezone)
| SqlLocalTimeOfDay TimeOfDay | Local HH:MM:SS (no timezone)
| SqlZonedLocalTimeOfDay TimeOfDay TimeZone | Local HH:MM:SS -HHMM. Converts to and from (TimeOfDay, TimeZone).
| SqlLocalTime LocalTime | Local YYYY-MM-DD HH:MM:SS (no timezone)
| SqlZonedTime ZonedTime | Local YYYY-MM-DD HH:MM:SS -HHMM. Considered equal if both convert to the same UTC time.
| SqlUTCTime UTCTime | UTC YYYY-MM-DD HH:MM:SS
| SqlDiffTime NominalDiffTime | Calendar diff between seconds. Rendered as Integer when converted to String, but greater precision may be preserved for other types or to underlying database.
| SqlPOSIXTime POSIXTime | Time as seconds since midnight Jan 1 1970 UTC. Integer rendering as for SqlDiffTime.
| SqlEpochTime Integer | DEPRECATED Representation of ClockTime or CalendarTime. Use SqlPOSIXTime instead.
| SqlTimeDiff Integer | DEPRECATED Representation of TimeDiff. Use SqlDiffTime instead.
| SqlNull | NULL in SQL or Nothing in Haskell
|
| Instances | |
|
|
|
Convert a value to an SqlValue. This function is simply
a restricted-type wrapper around convert. See extended notes on SqlValue.
|
|
|
Convert from an SqlValue to a Haskell value. Any problem is indicated by
calling error. This function is simply a restricted-type wrapper around
convert. See extended notes on SqlValue.
|
|
|
Conversions to and from SqlValues and standard Haskell types.
This function converts from an SqlValue to a Haskell value. Many people will use the simpler
fromSql instead. This function is simply a restricted-type wrapper around
safeConvert.
|
|
|
Converts any Integral type to a SqlValue by using toInteger.
|
|
|
Convenience function for using numeric literals in your program.
|
|
|
Convenience function for converting POSIXTime to a SqlValue, because
toSql cannot do the correct thing in this instance.
|
|
Database Connections
|
|
class IConnection conn where | Source |
|
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 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 any pending data to the database.
Required to make any changes take effect.
| | | Roll back to the state the database was in prior to the
last commit or rollback.
| | | 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.
| | | 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.
| | | 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.
| | | 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.
| | | 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.
| | | 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.
| | | 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.
| | | The version of the database server, if available.
| | | 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.
| | | 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.
| | | 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.
|
| | Instances | |
|
|
Wrapped Connections
|
|
|
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 | | Instances | |
|
|
|
Unwrap a ConnWrapper and pass the embedded IConnection to
a function. Example:
withWConn wrapped run $ "SELECT * from foo where bar = 1" []
|
|
Preparing Queries
|
|
|
Like run, but take a list of Maybe Strings instead of SqlValues.
|
|
|
Strict version of quickQuery.
|
|
|
A quick way to do a query. Similar to preparing, executing, and
then calling fetchAllRows on a statement. See also quickQuery'
|
|
Transaction Handling
|
|
This section concerns itself with writing (updating) a database.
In HDBC, as with many RDBMS implementations, every write to the
database occurs within a transaction. No changes are visible (outside
the current transaction) until a commit operation occurs, in which
case all changes since the transaction started are atomically
committed. Also, there is a rollback operation that can undo all
changes since the transaction started.
HDBC does everything within a transaction. A transaction is implicitly entered
when a connection to a database is established, and a transaction is
implicitly entered after each call to commit or rollback as well.
The practical effect of this is that you must call commit after making
changes to a database in order for those changes to become visible. You don't
have to call commit after every change, just after a batch of them.
(Exceptions exist for databases that don't offer a high level of transaction
isolation; but you should always play it safe and commit anyway.)
Database developers will also be experienced with the atomicity benefits
of transactions, an explanation of which is outside the scope of this manual.
Errors occuring at the database level can leave a transaction in an
indeterminate state, depending on the database. Some databases will
refuse all queries until the next commit or rollback. The safe thing
to do is to issue a commit or rollback after trapping any SqlError.
Alternatively, you could use withTransaction, which will automatically
handle this detail for you.
|
|
|
Execute some code. If any uncaught exception occurs, run
rollback and re-raise it. Otherwise, run commit and return.
This function, therefore, encapsulates the logical property that a transaction
is all about: all or nothing.
The IConnection object passed in is passed directly to the specified
function as a convenience.
This function traps all uncaught exceptions, not just SqlErrors. Therefore,
you will get a rollback for any exception that you don't handle. That's
probably what you want anyway.
Since all operations in HDBC are done in a transaction, this function doesn't
issue an explicit "begin" to the server. You should ideally have
called Database.HDBC.commit or Database.HDBC.rollback before
calling this function. If you haven't, this function will commit or rollback
more than just the changes made in the included action.
If there was an error while running rollback, this error will not be
reported since the original exception will be propogated back. (You'd probably
like to know about the root cause for all of this anyway.) Feedback
on this behavior is solicited.
|
|
Connection Inquiries
|
|
Statements
|
|
|
|
Execution
|
|
|
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.
|
|
|
Like execute, but take a list of Maybe Strings instead of
SqlValues.
|
|
|
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.
|
|
|
Like executeMany, but take a list of Maybe Strings instead of
SqlValues.
|
|
Fetching Results
|
|
|
Fetches one row from the DB. Returns Nothing if there
are no more rows. Will automatically call finish when
the last row is read.
|
|
|
Like fetchRow, but instead of returning a list, return an association
list from column name to value.
The keys of the column names are lowercase versions of the data returned
by getColumnNames. Please heed the warnings there. Additionally,
results are undefined if multiple columns are returned with identical names.
|
|
|
Similar to fetchRowAL, but return a Map instead of an association list.
|
|
|
Like fetchRow, but return a list of Maybe Strings instead of
SqlValues.
|
|
|
Lazily fetch all rows from an executed Statement.
You can think of this as hGetContents applied to a database result set.
The result of this is a lazy list, and each new row will be read, lazily, from
the database as the list is processed.
When you have exhausted the list, the Statement will be finished.
Please note that the careless use of this function can lead to some unpleasant
behavior. In particular, if you have not consumed the entire list, then
attempt to finish or re-execute the statement, and then attempt to consume
more elements from the list, the result will almost certainly not be what
you want.
But then, similar caveats apply with hGetContents.
Bottom line: this is a very convenient abstraction; use it wisely.
Use fetchAllRows' if you need something that is strict, without
all these caveats.
|
|
|
Strict version of fetchAllRows. Does not have the side-effects
of fetchAllRows, but forces the entire result set to be buffered
in memory.
|
|
|
Like fetchAllRows, but instead of returning a list for each
row, return an association list for each row, from column name to value.
See fetchRowAL for more details.
|
|
|
Strict version of fetchAllRowsAL
|
|
|
Like fetchAllRowsAL, but return a list of Maps instead of a list of
association lists.
|
|
|
Strict version of fetchAllRowsMap
|
|
|
Like fetchAllRows, but return Maybe Strings instead of SqlValues.
|
|
|
Strict version of sFetchAllRows.
|
|
|
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.
|
|
Statement Inquires
|
|
|
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.
|
|
Miscellaneous
|
|
|
Abort a query in progress -- usually not needed.
|
|
|
The original query that this Statement was prepared
with.
|
|
Exceptions
|
|
|
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 | | Instances | |
|
|
|
A utility function to throw a SqlError. The mechanics of throwing
such a thing differ between GHC 6.8.x, Hugs, and GHC 6.10. This function
takes care of the special cases to make it simpler.
With GHC 6.10, it is a type-restricted alias for throw. On all other systems,
it is a type-restricted alias for throwDyn.
|
|
|
Execute the given IO action.
If it raises a SqlError, then execute the supplied handler and return its
return value. Otherwise, proceed as normal.
|
|
|
Like catchSql, with the order of arguments reversed.
|
|
|
Given an Exception, return Just SqlError if it was an SqlError, or Nothing
otherwise. Useful with functions like catchJust.
|
|
|
Catches SqlErrors, and re-raises them as IO errors with fail.
Useful if you don't care to catch SQL errors, but want to see a sane
error message if one happens. One would often use this as a high-level
wrapper around SQL calls.
|
|
Column Types
|
|
These are defined in Database.HDBC.ColTypes but are
available to programs importing Database.HDBC by default as well.
See Database.HDBC.ColTypes for documentation.
|
|
module Database.HDBC.ColTypes |
|
Threading
|
|
FIXME: this is draft information
Thread support in a generalized interface such as HDBC can be complicated
because support for threading varies across database interfaces.
However, applications using HDBC should be able to rely upon at least a few
basic guarantees:
- The HDBC modules may freely be imported and used across all threads.
- HDBC modules may also freely share database connections and statements;
the database or HDBC driver will be responsible for locking if necessary.
I use "share" in the same sense as Python's DB-API: multiple threads may use
the resource without wrapping it in any lock.
However, there are some caveats to the above:
- Not all databases support more than one active statement for a single
connection. Therefore, for maximum portability, you should use
a different connection to the database for each simultaneous query you
wish to use.
FIXME: describe when a statement is active.
- Not all databases may support the level of multithreading described above.
For those that don't, safe access will be restriced in the HDBC driver
by using locks. Therefore, you can write portable code, but you
only get real multithreading when databases really support it.
Details of thread support should be documented in the HDBC
driver for each specific database.
|
|
Copyright and License
|
|
Copyright (C) 2005-2009 John Goerzen jgoerzen@complete.org
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
Please see COPYING in the source distribution for the full license.
|
|
Produced by Haddock version 2.4.2 |