Extensible Records

Version on this page:0.5.2
LTS Haskell 12.18:
Stackage Nightly 2018-11-16:0.10.0
Latest on Hackage:0.10.0

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MIT licensed by Jonathan Sterling

Extensible records for Haskell with lenses.



  • Changed the types of Data.Vinyl.CoRec.onCoRec and Data.Vinyl.CoRec.onField. This was pushing through the changes to drop the use of Proxy arguments, relying instead on TypeApplications. Also added onCoRec1 and onField to work with functions relying on a single type class.

  • Faster asA and asA'. These implementations utilize unsafeCoerce in their implementations after we have performed a runtime check that proves (to us) that the types match up. The old implementations are still available as asASafe and asA'Safe. While both implementations can run in constant time if the compiler optimizes everything successfully, the faster variants are a bit more than 3x faster in a trivial benchmark.

  • Add a Generic instance for Rec and common functors.

  • Add a variety of ToJSON implementations as a test case. One or all of these should probably exist as a separate package to avoid vinyl depending on aeson, but their content may be of interest.


  • Add runcurryX for applying an uncurried function to a Rec passing through the XRec machinery to strip out syntactic noise.


  • A new SRec type for constant time field access for records with densely packed Storable fields. Conversion from Rec is accomplished with toSRec, while fromSRec takes you back to Rec. Record updates are fairly slow compared to native Haskell records and even Rec, but reading a field is as fast as anything.

  • Concise record construction syntax from tuples. Construct a FieldRec with fieldRec (#x =: True, #y =: 'b') and have the type inferred as Rec ElField '[ '("x", Bool), '("y", Char) ]. Or use record to build records of any functor. Thanks to @heptahedron on GitHub for prompting this feature, and @sboosali for thinking through various approaches.

  • Optional concise record field lens syntax. This uses an orphan IsLabel instance for all function types, so will conflict with any other library that does the same. Thus it is entirely opt-in: to enable this syntax, you must explicitly import Data.Vinyl.Syntax. This enables the use of labels as lenses. For example, myRec & #name %~ map toUpper to apply map toUpper to the #name field of the record value myRec. This technique is thanks to Tikhon Jelvis who shared it on the Haskell-Cafe mailing list.

  • Field lenses can now change the type of a record. Thanks to @heptahedron on GitHub for exploring this feature. Using the above-mentioned features, one might now write something like myRec & #name %~ length to produce a record whose #name field is the length of theString #name field of some record value, myRec.

  • Changed the type of =:= again to work directly with Labels as this is the most convenient usage.

  • Definitions in Data.Vinyl.Core are now consistently in terms of type classes. This permits inlining and specialization to a user’s record types. In the case where the record type is known, call sites do not change. But for functions polymorphic in the record’s fields, a constraint will be required. If those constraints are a nuisance, or compile times increase beyond comfort, users should use definitions from the Data.Vinyl.Recursive that are written in a recursive style (as in previous versions of the vinyl package), treating the record as a list of fields.

  • Added restrictCoRec and weakenCoRec suggested by @ElvishJerricco


  • Overhaul of FieldRec: records with named fields. We now take advantage of the -XOverloadedLabels extension to support referring to record fields by names such a #myField.

  • A new ARec type for constant-time field access. You can convert a classic, HList-like Rec into an ARec with toARec, or back the other way with fromARec. An ARec uses an Array to store record fields, so the usual trade-offs between lists and arrays apply: lists are cheap to construct by adding an element to the head, but slow to access; it is expensive to modify the shape of an array, but element lookup is constant-time.

Compatibility Break: The operator =: for constructing a record with a single field has changed. That operation is now known as =:=, while =: is now used to construct an ElField. It was decided that single-field record construction was not a common use-case, so the shorter name could be used for the more common operation. Apologies for making the upgrade a bit bumpy.


  • Simplified match
  • Added Data.Vinyl.Curry


Added a CoRec (co-record) type constructed in the same style as the existing Rec type for records. A CoRec is an open sum type: a value of CoRec [a,b,c] is either an a, a b, or a c. In contrast a Rec [a,b,c] includes an a, a b, and, a c.


Added a concise Show instance for Const.


Ported the tutorial to haddocks (andrewthad)


Added utilities for working with the FieldRec type.


Vinyl 0.5 combines the generality of Vinyl 0.4 with the ease-of-use of previous versions by eschewing the defunctionalized type families and just using plain type constructors; Vinyl 0.4-style records can be recovered in most cases in a modular manner without baking it into the fabric of Vinyl itself.

Also new in 0.5 is a unified lens-based approach to subtyping, coercion and projection.


Vinyl 0.4 is a big departure from previous versions, in that it introduces a universe encoding as a means to generalize the space of keys from strings to any arbitrary space. This means that you can have closed universes for your records.

For details on how to use the new Vinyl, please see tests/Intro.lhs or view Jon’s talk at BayHac 2014, Programming in Vinyl.

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