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

Example type specialization:

runSelect :: Select (Field SqlInt4, Field SqlText) -> IO [(Int, String)]

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

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

Version of runSelect with better type inference

runSelectTF has better type inference than runSelect but only works with "higher-kinded data" types.

runSelectFold 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.

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

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.

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

newtype Foo = Foo Int

instance DefaultFromField Foo Foo where
   defaultFromField = unsafeFromField Foo defaultFromField

It is "unsafe" because it does not check that the sqlType correctly corresponds to the Haskell type.

Cursor within a transaction.

A FromFields specifies how to convert Postgres values (fields) 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 FromFields instance.

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

A FromField sqlType haskellType encodes how to turn a value of Postgres type sqlType into a value of Haskell type haskellType. For example a value of type FromField SqlText String encodes how to turn a SqlText result from the database into a Haskell String.

"FromField sqlType haskellType" corresponds to postgresql-simple's "FieldParser haskellType".

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

"DefaultFromField sqlType haskellType" corresponds to postgresql-simple's "FromField haskellType".

Creating an instance of DefaultFromField 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 DefaultFromField instance.

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