OverloadedRecordFields

The Overloaded Record Fields family of extensions for GHC allow multiple record datatypes to share the same field names, and make it possible for type information to disambiguate selectors. In the design as implemented, there is no single OverloadedRecordFields extension, but there are extensions for DuplicateRecordFields and OverloadedLabels.

For user-facing documentation, see the GHC user's guide:

• DuplicateRecordFields extension

• Record field selector polymorphism (HasField class)

For design details and implementation status, see:

• #18598: tracking ticket for most recent work related to overloaded record fields.

• GHC proposals:

  • Adding setField to HasField (2018, being implemented for GHC 9.2, see #16232)
  • Adding HasField class, changes to OverloadedLabels (2016, implemented in GHC 8.2 without IsLabel x (r -> a) instance)

• Redesigned variant involving three extensions (2015)

  • Part 1: DuplicateRecordFields (in GHC 8.0)
  • Part 2: OverloadedLabels (in GHC 8.0)
  • Part 3: Magic type classes (partly in GHC 8.2)
  • Adam Gundry's blog post

• Original design (2013)

Content previously on this page has been moved to the SORF page.

Issues

See the OverloadedRecordFields label.

Code

• Prototype implementation of the magic typeclasses
• Phab:D761, Phab:D1391, Phab:D1486, Phab:D1586, Phab:D1600: DuplicateRecordFields extension
• Phab:D1331, Phab:D1623: OverloadedLabels extension
• Phab:D1687, Phab:D2708: magic classes
• !3257: extension of HasField class to support updates

History

The extension was initially implemented in 2013 as a Google Summer of Code project, by Adam Gundry under the mentorship of Simon Peyton Jones.

• Simple Overloaded Record Fields (SORF), Simon PJ's original proposal
• Declared Overloaded Record Fields (DORF), a counterpoint proposal by Anthony Clayden
• Original design of the extension
• Discussion of the problem and possible solutions
• Google Summer of Code project details