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overloaded record fields

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 fields. There is no single OverloadedRecordFields extension, but rather a family of related extensions:

  • DisambiguateRecordFields: makes use of constructor names to disambiguate fields in record construction or pattern matching.
  • DuplicateRecordFields: permits a module to define the same field name in multiple datatypes.
  • OverloadedLabels: provides #foo syntax for an identifier whose meaning is determined by typeclass instance resolution. Intended for use with the HasField magic type class.
  • NoFieldSelectors: prevents fields being in scope as selector functions. (Not yet released as of January 2020.)
  • RecordDotSyntax: permits expression.field syntax for record projection. (Not yet released as of January 2020.)

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

For implementation status, see #18598, the tracking ticket for most recent work related to overloaded record fields, and the OverloadedRecordFields label.

For design discussion, see the GHC proposals:

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

Code

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.