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 } 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.