Database.HDBC.SqlValue

HDBC-2.3.1.2: Haskell Database Connectivity

Safe Haskell | None

Database.HDBC.SqlValue

Contents

- SQL value marshalling

Synopsis

- data SqlValue
  - = SqlString String
  - | SqlByteString ByteString
  - | SqlWord32 Word32
  - | SqlWord64 Word64
  - | SqlInt32 Int32
  - | SqlInt64 Int64
  - | SqlInteger Integer
  - | SqlChar Char
  - | SqlBool Bool
  - | SqlDouble Double
  - | SqlRational Rational
  - | SqlLocalDate Day
  - | SqlLocalTimeOfDay TimeOfDay
  - | SqlZonedLocalTimeOfDay TimeOfDay TimeZone
  - | SqlLocalTime LocalTime
  - | SqlZonedTime ZonedTime
  - | SqlUTCTime UTCTime
  - | SqlDiffTime NominalDiffTime
  - | SqlPOSIXTime POSIXTime
  - | SqlEpochTime Integer
  - | SqlTimeDiff Integer
  - | SqlNull
- safeFromSql :: Convertible SqlValue a => SqlValue -> ConvertResult a
- toSql :: Convertible a SqlValue => a -> SqlValue
- fromSql :: Convertible SqlValue a => SqlValue -> a
- nToSql :: Integral a => a -> SqlValue
- iToSql :: Int -> SqlValue
- posixToSql :: POSIXTime -> SqlValue

# SQL value marshalling

data SqlValue

`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 `SqlValue`s 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

Eq SqlValue |
Show SqlValue |
Typeable SqlValue |
Convertible Bool SqlValue |
Convertible Char SqlValue |
Convertible Double SqlValue |
Convertible Int SqlValue |
Convertible Int32 SqlValue |
Convertible Int64 SqlValue |
Convertible Integer SqlValue |
Convertible Rational SqlValue |
Convertible Word32 SqlValue |
Convertible Word64 SqlValue |
Convertible String SqlValue |
Convertible ByteString SqlValue |
Convertible ByteString SqlValue |
Convertible ClockTime SqlValue |
Convertible CalendarTime SqlValue |
Convertible TimeDiff SqlValue |
Convertible Text SqlValue |
Convertible Text SqlValue |
Convertible LocalTime SqlValue |
Convertible ZonedTime SqlValue |
Convertible TimeOfDay SqlValue |
Convertible UTCTime SqlValue |
Convertible NominalDiffTime SqlValue |
Convertible Day SqlValue |
Convertible DiffTime SqlValue |
Convertible SqlValue Bool |
Convertible SqlValue Char |
Convertible SqlValue Double |
Convertible SqlValue Int |
Convertible SqlValue Int32 |
Convertible SqlValue Int64 |
Convertible SqlValue Integer |
Convertible SqlValue Rational |
Convertible SqlValue Word32 |
Convertible SqlValue Word64 |
Convertible SqlValue String |
Convertible SqlValue ByteString |
Convertible SqlValue ByteString |
Convertible SqlValue ClockTime |
Convertible SqlValue CalendarTime |
Convertible SqlValue TimeDiff |
Convertible SqlValue Text |
Convertible SqlValue Text |
Convertible SqlValue LocalTime |
Convertible SqlValue ZonedTime |
Convertible SqlValue TimeOfDay |
Convertible SqlValue UTCTime |
Convertible SqlValue NominalDiffTime |
Convertible SqlValue Day |
Convertible SqlValue DiffTime |
Convertible SqlValue SqlValue |
Convertible SqlValue a => Convertible SqlValue (Maybe a) |
Convertible SqlValue (TimeOfDay, TimeZone) |
Convertible a SqlValue => Convertible (Maybe a) SqlValue |
Convertible (TimeOfDay, TimeZone) SqlValue |

safeFromSql :: Convertible SqlValue a => SqlValue -> ConvertResult a

Conversions to and from `SqlValue`s 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`.

toSql :: Convertible a SqlValue => a -> SqlValue

Convert a value to an `SqlValue`. This function is simply a restricted-type wrapper around `convert`. See extended notes on `SqlValue`.

fromSql :: Convertible SqlValue a => SqlValue -> a

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`.

nToSql :: Integral a => a -> SqlValue

Converts any Integral type to a `SqlValue` by using toInteger.

iToSql :: Int -> SqlValue

Convenience function for using numeric literals in your program.

posixToSql :: POSIXTime -> SqlValue

Convenience function for converting `POSIXTime` to a `SqlValue`, because `toSql` cannot do the correct thing in this instance.