# fclabels: First class accessor labels implemented as lenses.

[ bsd3, data, lenses, library ] [ Propose Tags ]

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
- Minor documentaion update.
- Exported Point internals.
- Renamed Label to Lens.

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