Documentation

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 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 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.

Cursor within a transaction.