RecordFromSql q a is data-type wrapping function to convert from list of database value type (to receive from database) [q] into Haskell type a

This structure is similar to parser. While running RecordFromSql behavior is the same as non-fail-able parser which parse list of database value type [q] stream.

So, RecordFromSql q is Monad and Applicative instance like parser monad. When, you have data constructor and objects like below.

  data MyRecord = MyRecord Foo Bar Baz

  foo :: RecordFromSql SqlValue Foo
  foo =  ...
  bar :: RecordFromSql SqlValue Bar
  bar =  ...
  baz :: RecordFromSql SqlValue Baz
  baz =  ...

You can get composed RecordFromSql like below.

  myRecord :: RecordFromSql SqlValue MyRecord
  myRecord =  MyRecord <$> foo <*> bar <*> baz

Run RecordFromSql parser function object. Convert from list of database value type [q] into Haskell type a and rest of list [q].

Run RecordFromSql parser function object. Convert from list of database value type [q] into Haskell type a.

Axiom of RecordFromSql for database value type q and Haskell type a

Derivation rule of RecordFromSql parser function object for Haskell tuple (,) type.

Derivation rule of RecordFromSql parser function object for Haskell Maybe type.

FromSql q a is implicit rule to derive RecordFromSql q a record parser function against type a.

Generic programming (https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#generic-programming) with default signature is available for FromSql class, so you can make instance like below:

  {-# LANGUAGE DeriveGeneric #-}
  import GHC.Generics (Generic)
  import Database.HDBC (SqlValue)
  --
  data Foo = Foo { ... } deriving Generic
  instance FromSql SqlValue Foo

Run implicit RecordFromSql parser function object. Convert from list of database value type [q] into haskell type a and rest of list [q].

Run implicit RecordFromSql parser function object. Convert from list of database value type [q] into haskell type a.

Derivation rule of RecordFromSql parser function object for value convert function.