runQuery's use of the Default typeclass means that the compiler will have trouble inferring types. It is strongly recommended that you provide full type signatures when using runQuery.

Example type specialization:

runQuery :: Select (Column SqlInt4, Column SqlText) -> IO [(Int, String)]

Assuming the makeAdaptorAndInstance splice has been run for the product type Foo:

runQuery :: Select (Foo (Column SqlInt4) (Column SqlText) (Column SqlBool)
         -> IO [Foo Int String Bool]

Opaleye types are converted to Haskell types based on instances of the QueryRunnerColumnDefault typeclass.

runQueryFold streams the results of a query incrementally and consumes the results with a left fold.

This fold is not strict. The stream consumer is responsible for forcing the evaluation of its result to avoid space leaks.

Use queryRunnerColumn to make an instance to allow you to run queries on your own datatypes. For example:

newtype Foo = Foo Int

instance QueryRunnerColumnDefault Foo Foo where
   queryRunnerColumnDefault =
       queryRunnerColumn (unsafeCoerceColumn
                              :: Column Foo -> Column SqlInt4)
                         Foo
                         queryRunnerColumnDefault

Declare a temporary cursor. The cursor is given a unique name for the given connection.

Returns Nothing when the query returns zero rows.

Like declareCursor but takes a FromFields explicitly.

Close the given cursor.

Fold over a chunk of rows, calling the supplied fold-like function on each row as it is received. In case the cursor is exhausted, a Left value is returned, otherwise a Right value is returned.

Cursor within a transaction.

A QueryRunner specifies how to convert Postgres values (columns) into Haskell values (haskells). Most likely you will never need to create on of these or handle one directly. It will be provided for you by the Default QueryRunner instance.

"QueryRunner columns haskells" corresponds to postgresql-simple's "RowParser haskells". "Default QueryRunner columns haskells" corresponds to postgresql-simple's "FromRow haskells".

A QueryRunnerColumn pgType haskellType encodes how to turn a value of Postgres type pgType into a value of Haskell type haskellType. For example a value of type QueryRunnerColumn PGText String encodes how to turn a PGText result from the database into a Haskell String.

"QueryRunnerColumn pgType haskellType" corresponds to postgresql-simple's "FieldParser haskellType".

A QueryRunnerColumnDefault pgType haskellType represents the default way to turn a pgType result from the database into a Haskell value of type haskellType.

"QueryRunnerColumnDefault pgType haskellType" corresponds to postgresql-simple's "FromField haskellType".

Creating an instance of QueryRunnerColumnDefault for your own types is necessary for retrieving those types from the database.

You should use one of the three methods below for writing a QueryRunnerColumnDefault instance.

1. If you already have a FromField instance for your haskellType, use fieldQueryRunnerColumn. (This is how most of the built-in instances are defined.)
2. If you don't have a FromField instance, use queryRunnerColumn if possible. See the documentation for queryRunnerColumn for an example.
3. If you have a more complicated case, but not a FromField instance, write a FieldParser for your type and use fieldParserQueryRunnerColumn. You can also add a FromField instance using this.