postgresql-simple-0.1: Mid-Level PostgreSQL client library

MaintainerLeon P Smith <>
Safe HaskellSafe-Infered



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.



class FromField a whereSource

A type that may be converted from a SQL type.


fromField :: Field -> Maybe ByteString -> Ok aSource

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.

data ResultError Source

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



The SQL and Haskell types are not compatible.


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


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).

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

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.

data Field Source

A Field represents metadata about a particular field

You don't particularly want to retain these structures for a long period of time, as they will retain the entire query result, not just the field metadata

newtype Oid


Oid CUInt 

data Format