HDBC- Haskell Database Connectivity

Safe HaskellSafe-Infered




SQL value marshalling

data SqlValue Source

SqlValue is he 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 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.


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.


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.


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.


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.


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.


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.


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.


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.


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


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.


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.


NULL in SQL or Nothing in Haskell


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 aSource

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.

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

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 -> aSource

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 -> SqlValueSource

Converts any Integral type to a SqlValue by using toInteger.

iToSql :: Int -> SqlValueSource

Convenience function for using numeric literals in your program.

posixToSql :: POSIXTime -> SqlValueSource

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