Module documentation for 0.10.0.1
Extensible records for Haskell with lenses.
- Changed the
- Added the
corechelper that specifically helps type inference when constructing
Changed the types of
Data.Vinyl.CoRec.onField. This was pushing through the changes to drop the use of
Proxyarguments, relying instead on
TypeApplications. Also added
onFieldto work with functions relying on a single type class.
asA'. These implementations utilize
unsafeCoercein 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
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.
Recand common functors.
Add a variety of
ToJSONimplementations as a test case. One or all of these should probably exist as a separate package to avoid
aeson, but their content may be of interest.
runcurryXfor applying an uncurried function to a
Recpassing through the
XRecmachinery to strip out syntactic noise.
SRectype for constant time field access for records with densely packed
Storablefields. Conversion from
Recis accomplished with
fromSRectakes 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 (#x =: True, #y =: 'b')and have the type inferred as
Rec ElField '[ '("x", Bool), '("y", Char) ]. Or use
recordto 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
IsLabelinstance 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 toUpperto apply
map toUpperto the
#namefield 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 %~ lengthto produce a record whose
#namefield is the length of the
#namefield of some record value,
Changed the type of
=:=again to work directly with
Labels as this is the most convenient usage.
Data.Vinyl.Coreare 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.Recursivethat are written in a recursive style (as in previous versions of the
vinylpackage), treating the record as a list of fields.
weakenCoRecsuggested by @ElvishJerricco
FieldRec: records with named fields. We now take advantage of the
-XOverloadedLabelsextension to support referring to record fields by names such a
ARectype for constant-time field access. You can convert a classic, HList-like
toARec, or back the other way with
Arrayto 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
=: 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.
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
b, or a
c. In contrast a
Rec [a,b,c] includes an
b, and, a
Added a concise
Show instance for
Ported the tutorial to haddocks (andrewthad)
Added utilities for working with the
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