fclabels-0.9.1: fclabels.cabal
Name: fclabels
Version: 0.9.1
Author: Sebastiaan Visser, Erik Hesselink, Chris Eidhof, Sjoerd Visscher.
Synopsis: First class accessor labels implemented as lenses.
Description: First class labels that act as bidirectional record fields.
.
The labels are implemented as lenses and are fully composable
and can be used to get, set and modify parts of a datatype in
a consistent way. The lens datatype, conveniently called
`:->', is an instance of the `Category' type class: meaning it
has a proper identity and composition. The library has support
for automatically deriving labels from record selectors that
start with an underscore. Labels can be used in a purely
functional setting or be applied to mutable state in some
state monad.
.
To illustrate this package, let's take the following two example
datatypes (somehow Haddock removes the curly braces):
.
> data Person = Person {
> _name :: String
> , _age :: Int
> , _isMale :: Bool
> , _place :: Place
> }
.
> data Place = Place {
> _city
> , _country
> , _continent :: String
> }
.
Both are record datatypes with all record labels prefixed by
an underscore. This underscore is an indication for our
Template Haskell code to derive lenses for these fields.
Deriving lenses can be done with this simple one-liner:
.
> $(mkLenses [''Person, ''Place])
.
These lenses can be used to get, set and modify the value and
are fully composable.
.
Now let's look at this example. This 71 year old fellow, called Jan,
is my neighbour and didn't mind using him as an example:
.
> jan :: Person
> jan = Person "Jan" 71 True (Place "Utrecht" "The Netherlands" "Europe")
.
When we want to be sure Jan is really as old as he claims we
can use the @getL@ function to get the age out as an integer:
.
> hisAge :: Int
> hisAge = getL age jan
.
Consider he now wants to move to Amsterdam: what better place
to spend your old days. Using composition we can change the
city value deep inside the structure:
.
> moveToAmsterdam :: Person -> Person
> moveToAmsterdam = setL (city . place) "Amsterdam"
.
> moveToAmsterdam jan ==
> Person "Jan" 71 True (Place "Amsterdam" "The Netherlands" "Europe")
.
Composition is done using the dot operator which is part of
the @Control.Category@ module. Make sure to import this module
and hide the default @(.)@, @id@ and @modL@ function from the
Prelude.
.
Now, because Jan is an old guy, moving to another city is not a
very easy task, this really takes a while. It will probably
take no less than two years before he will actually be
settled. To reflect this change it might be useful to have a
first class view on the @Person@ data type that only reveals
the age and city. This can be done by using a neat
@Applicative@ functor instance:
.
> ageAndCity :: Person :-> (Int, String)
> ageAndCity = Lens $ (,) <$> fst `for` age <*> snd `for` (city . place)
.
Because the applicative type class on its own is not very
capable of expressing bidirectional relations, which we need
for our lenses, the actual instance is defined for an internal
helper structure called @Point@. Points are a bit more general
than lenses. As you can see above, the @Label@ constructor has
to be used to convert a @Point@ back into a @Label@. The @for@
function must be used to indicate which partial destructor to
use for which lens in the applicative composition.
.
Now that we have an appropriate age+city view on the @Person@
data type (which is itself a lens again), we can use the
@modL@ function to make Jan move to Amsterdam over exactly two
years:
.
> moveToAmsterdamOverTwoYears :: Person -> Person
> moveToAmsterdamOverTwoYears = modL ageAndCity (\(a, b) -> (a+2, "Amsterdam"))
.
> moveToAmsterdamOverTwoYears jan ==
> Person "Jan" 73 True (Place "Amsterdam" "The Netherlands" "Europe")
.
This package also contains a lens data type that encodes
bidirectional functions. Just like lenses, lenses can be
composed with other lenses using the @Control.Category@ type
class. Lenses can be used to change the type of a lens. The
@Iso@ type class, which can be seen as a bidirectional
functor, can be used to apply lenses to lenses. For example,
when we want to treat the age of a person as a string we can
do the following:
.
> ageAsString :: Person :-> String
> ageAsString :: (show :<->: read) % age
.
A final note: this library might look cryptic at first sight, but give it a
try, it is not that hard.
.
.
> CHANGELOG
> 0.4.2 -> 0.9.1
> - Added askM and localM for running lenses inside MonadReader.
> - Minor documentaion update.
> - Exported Point internals.
> - Renamed Label to Lens.
Maintainer: Sebastiaan Visser <haskell@fvisser.nl>
License: BSD3
License-File: LICENCE
Category: Data
Build-Type: Simple
Cabal-Version: >= 1.6
HS-Source-Dirs: src
Exposed-Modules: Data.Record.Label
Other-Modules: Data.Record.Label.TH
Build-Depends: base >= 3 && < 5
, template-haskell >= 2.2 && < 2.5
, monads-fd >= 0.0 && < 0.2