Database.PostgreSQL.Simple.FromRow

The FromRow typeclass, for converting a row of results returned by a SQL query into a more useful Haskell representation.

Predefined instances are provided for tuples containing up to ten elements.

Documentation

class FromRow a where

A collection type that can be converted from a sequence of fields. Instances are provided for tuples up to 10 elements and lists of any length.

Note that instances can defined outside of postgresql-simple, which is often useful. For example, here's an instance for a user-defined pair:

data User = User { name :: String, fileQuota :: Int }

instance FromRow User where
     fromRow = User <$> field <*> field

The number of calls to field must match the number of fields returned in a single row of the query result. Otherwise, a ConversionFailed exception will be thrown.

Note that field evaluates it's result to WHNF, so the caveats listed in previous versions of postgresql-simple no longer apply. Instead, look at the caveats associated with user-defined implementations of fromRow.

Methods

fromRow :: RowParser a

Instances

FromField a => FromRow [a]

FromField a => FromRow (Only a)

(FromField a, FromField b) => FromRow (a, b)

(FromRow a, FromRow b) => FromRow (:. a b)

(FromField a, FromField b, FromField c) => FromRow (a, b, c)

(FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d)

(FromField a, FromField b, FromField c, FromField d, FromField e) => FromRow (a, b, c, d, e)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f) => FromRow (a, b, c, d, e, f)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g) => FromRow (a, b, c, d, e, f, g)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h) => FromRow (a, b, c, d, e, f, g, h)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i) => FromRow (a, b, c, d, e, f, g, h, i)

(FromField a, FromField b, FromField c, FromField d, FromField e, FromField f, FromField g, FromField h, FromField i, FromField j) => FromRow (a, b, c, d, e, f, g, h, i, j)

data RowParser a

Instances

Monad RowParser

Functor RowParser

Applicative RowParser

Alternative RowParser

field :: FromField a => RowParser a

fieldWith :: FieldParser a -> RowParser a

numFieldsRemaining :: RowParser Int