Database.PostgreSQL.Simple.Result

The Result 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 PostgreSQL numeric types that are less accurate than it. For instance, the Haskell Double type is compatible with the PostgreSQL'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 Result a where

A type that may be converted from a SQL type.

Methods

convert :: Field -> Maybe ByteString -> Either SomeException a

Convert a SQL value to a Haskell value.

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

Instances

Result Bool

Result Double

Result Float

Result Int

Result Int16

Result Int32

Result Int64

Result Integer

Result ByteString

Result Text

Result Text

Result ByteString

Result Oid

Result TimeOfDay

Result UTCTime

Result Day

Result Null

Result RawResult

Result [Char]

Result (Ratio Integer)

Result a => Result (Maybe a)

Result (Binary ByteString)

Result (Binary ByteString)

(Result a, Result b) => Result (Either a b)

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

Typeable ResultError

Exception ResultError