Database.SQLite.Simple.FromField

The FromField typeclass, for converting a single value in a row returned by a SQL query into a more useful Haskell representation.

A Haskell numeric type is considered to be compatible with all SQLite numeric types that are less accurate than it. For instance, the Haskell Double type is compatible with the SQLite's 32-bit Int type because it can represent a Int exactly. On the other hand, since a Double might lose precision if representing a 64-bit BigInt, the two are not considered compatible.

Documentation

class FromField a where

A type that may be converted from a SQL type.

Methods

fromField :: FieldParser a

Convert a SQL value to a Haskell value.

Returns a list of exceptions if the conversion fails. In the case of library instances, this will usually be a single ResultError, but may be a UnicodeException.

Implementations of fromField should not retain any references to the Field nor the ByteString arguments after the result has been evaluated to WHNF. Such a reference causes the entire LibPQ.Result to be retained.

For example, the instance for ByteString uses copy to avoid such a reference, and that using bytestring functions such as drop and takeWhile alone will also trigger this memory leak.

Instances

FromField Bool

FromField Double

FromField Float

FromField Int

FromField Int16

FromField Int32

FromField Int64

FromField Integer

FromField ByteString

FromField ByteString

FromField Text

FromField Text

FromField UTCTime

FromField Day

FromField Null

FromField [Char]

FromField a => FromField (Maybe a)

type FieldParser a = Field -> Ok a

data ResultError

Exception thrown if conversion from a SQL value to a Haskell value fails.

Constructors

Incompatible

The SQL and Haskell types are not compatible.

Fields

errSQLType :: String

errHaskellType :: String

errMessage :: String

UnexpectedNull

A SQL NULL was encountered when the Haskell type did not permit it.

Fields

errSQLType :: String

errHaskellType :: String

errMessage :: String

ConversionFailed

The SQL value could not be parsed, or could not be represented as a valid Haskell value, or an unexpected low-level error occurred (e.g. mismatch between metadata and actual data in a row).

Fields

errSQLType :: String

errHaskellType :: String

errMessage :: String

Instances

Eq ResultError

Show ResultError

Exception ResultError

Typeable * ResultError

data Field

A Field represents metadata about a particular field

fieldData :: Field -> SQLData

Return the actual SQL data for a database field. This allows user-defined FromField instances to access the SQL data associated with a field being parsed.

returnError :: forall a err. (Typeable a, Exception err) => (String -> String -> String -> err) -> Field -> String -> Ok a

Given one of the constructors from ResultError, the field, and an errMessage, this fills in the other fields in the exception value and returns it in a 'Left . SomeException' constructor.