Portability | portable |
---|---|
Stability | experimental |
Maintainer | Leon P Smith <leon@melding-monads.com> |
Safe Haskell | Safe-Infered |
Database.PostgreSQL.Simple.FromRow
Description
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
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 } instanceFromRow
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
.
Instances