The FromField typeclass, for converting a single value in a row returned by a SQL query into a more useful Haskell representation. Note that each instance of FromField is documented by a list of compatible postgresql types.

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 PostgreSQL's 64-bit bigint, the two are not considered compatible.

Note that the Float and Double instances use attoparsec's double conversion routine, which sacrifices some accuracy for speed. If you need accuracy, consider first converting data to a Scientific or Rational type, and then converting to a floating-point type. If you are defining your own FromRow instances, this can be achieved simply by fromRational <$> field, although this idiom is additionally compatible with PostgreSQL's int8 and numeric types. If this is unacceptable, you may find fieldWith useful.

Also note that while converting to a Double through the Scientific type is likely somewhat faster than converting through the Rational type, the Scientific type has no way to represent NaN and ±Infinity values. Thus, if you need precision conversion of regular floating point values and the possibility of receiving these special values from the backend, stick with Rational.

Because FromField is a typeclass, one may provide conversions to additional Haskell types without modifying postgresql-simple. This is particularly useful for supporting PostgreSQL types that postgresql-simple does not support out-of-box. Here's an example of what such an instance might look like for a UUID type that implements the Read class:

import Data.UUID ( UUID )
import Database.PostgreSQL.Simple.FromField
       ( FromField (fromField) , typeOid, returnError, ResultError (..) )
import Database.PostgreSQL.Simple.TypeInfo.Static (typoid, uuid)
import qualified Data.ByteString.Char8 as B

instance FromField UUID where
   fromField f mdata =
      if typeOid f /= typoid uuid
        then returnError Incompatible f ""
        else case B.unpack `fmap` mdata of
               Nothing  -> returnError UnexpectedNull f ""
               Just dat ->
                  case [ x | (x,t) <- reads dat, ("","") <- lex t ] of
                    [x] -> return x
                    _   -> returnError ConversionFailed f dat

Note that because PostgreSQL's uuid type is built into postgres and is not provided by an extension, the typeOid of uuid does not change and thus we can examine it directly. One could hard-code the type oid, or obtain it by other means, but in this case we simply pull it out of the static table provided by postgresql-simple.

On the other hand if the type is provided by an extension, such as PostGIS or hstore, then the typeOid is not stable and can vary from database to database. In this case it is recommended that FromField instances use typename instead.