http://hackage.haskell.org/trac/ghc/wiki/ExtensibleRecords
I'm happy to see a Wiki page to summarise and contrast different approaches; that seems like a constructive >thing to do. (Email discussions tend to evaporate and then be repeated.) A useful thing to do would be to >give a series of use-cases, or examples, showing the kinds of thing one would like to be able to do. Then you >can classify the approaches by what examples they can handle.
i've added a link to the current version of my Data.Record (previously listed as "Poor Man's Records), which supports just about all operations that have been mentioned so far, including scoped labels, Has/Lacks, or the generalised extension-safe operations and predicates Barney prefers. and we still do not need the order on the field labels that plagues most other extensible record libraries (i'm quite amazed myself, and expect bug reports!-). i've also added the three language and implementation features i consider most important to make library-based extensible records useable in practice (none of which happen to be specific to records, btw;-). in trying to be comprehensive (mostly to explore the limitations of my approach), my code is not as simple and clean as the type families version. i don't want to duplicate the in-file comments on the wiki page for maintenance reasons, but i append them below, for the curious. claus {- poor man's extensible and concatenable records supporting both scoped and unique label records class Label label | record field labels label := value | record fields predicates: rec `Lacks` label | lacks predicate: record lacks field label rA `Disjoint` rB | record disjointness: records share no field labels scoped records operations: field :# rec | record extension rec #? label | field selection rec #- label | field removal (rec #! label) val | field update: update field label with value (rec #@ old) new | field renaming: rename existing field label old to new recA ## recB | symmetric record concatenation recA #^ recB | left-biased record field type intersection recA #^^ recB | left-biased record field label intersection recA #& recB | record projection: for each recB field, select matching recA field other record operations: field !:# rec | strict record extension: no duplicate field labels rec !#- label | strict field removal: remove existing field label from rec (rec !#! label) val | strict field update: update existing field label with value recA !## recB | symmetric disjoint record concatenation recA !#& recB | strict record projection: permute recA to match recB types: class Label l where label :: l class Has label rec lbool | label rec -> lbool class Lacks rec label class Disjoint recA recB label :: (Label l) => l (:=) :: label -> value -> label := value (:#) :: field -> record -> field :# record (!:#) :: (Lacks rec label) => (label := val) -> rec -> (label := val) :# rec (#?) :: (Select label val rec) => rec -> label -> val (#-) :: (Remove label rec rec') => rec -> label -> rec' (!#-) :: (Has label rec LTrue, Remove label rec rec') => rec -> label -> rec' (#!) :: (Remove label rec rec') => rec -> label -> value -> (label := value) :# rec' (!#!) :: (Has label rec LTrue, Remove label rec rec') => rec -> label -> value -> (label := value) :# rec' (#@) :: (Remove label1 rec rec', Select label1 val rec) => rec -> label -> label1 -> (label := val) :# rec' (##) :: (Concat recA recB recAB) => recA -> recB -> recAB (!##) :: (recA `Disjoint` recB, Concat recA recB recAB) => recA -> recB -> recAB (#^) :: (Intersect recA recB recAB) => recA -> recB -> recAB (#^^) :: (Intersect' recA recB recAB) => recA -> recB -> recAB (#&) :: (Project recA recB) => recA -> recB -> recB (!#&) :: (Project' recA recB) => recA -> recB -> recB see main at the bottom for examples of use. please let me know of any practically relevant missing operations Claus Reinke February 2006: submitted to support proposal for first class labels in Haskell' November 2007: added many more operations and predicates, replaced pairs with symbolic constructors, added strict operations -}