Framework for record merging.

MergeableRecord is intended to be a super class for classes providing update operations on records. The ADT induced by such a set of operations can be considered a "mergeable record", which can be merged into larger mergeable records essentially by function composition. Finalization turns a mergeable record into a record.

Typical use:

Given

 data Foo = Foo {l1 :: T1, l2 :: T2}

one define a mergeable record type (MR Foo) by the following instance:

  instance MergeableRecord Foo where
      mrDefault = Foo {l1 = v1_dflt, l2 = v2_dflt}

Typically, one would also provide definitions for setting the fields, possibly (but not necessarily) overloaded:

  instance HasL1 Foo where
      setL1 v = mrMake (foo -> foo {l1 = v})

Now Foo records can be created as follows:

  let foo1 = setL1 v1
  ...
  let foo2 = setL2 v2 ~+~ foo1
  ...
  let fooN = setL1 vN ~+~ fooN-1
  let fooFinal = mrFinalize fooN