Data.Label

This package provides first class labels that can act as bidirectional record fields. The labels can be derived automatically using Template Haskell which means you don't have to write any boilerplate yourself. The labels are implemented as lenses and are fully composable. Labels can be used to get, set and modify parts of a datatype in a consistent way.

Working with fclabels.

The lens datatype, conveniently called :->, is an instance of the Control.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.

To illustrate this package, let's take the following two example datatypes.

{-# LANGUAGE TemplateHaskell, TypeOperators #-}
import Control.Category
import Data.Label
import Prelude hiding ((.), id)

data Person = Person
  { _name   :: String
  , _age    :: Int
  , _isMale :: Bool
  , _place  :: Place
  } deriving Show

data Place = Place
  { _city
  , _country
  , _continent :: String
  } deriving Show

Both datatypes are record types with all the labels prefixed with 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:

mkLabels [''Person, ''Place]

For all labels a lens will created.

Now let's look at this example. This 71 year old fellow, my neighbour called Jan, 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 get function to get the age out as an integer:

hisAge :: Int
hisAge = get 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 = set (city . place) "Amsterdam"

And now:

ghci> moveToAmsterdam jan
Person "Jan" 71 True (Place "Amsterdam" "The Netherlands" "Europe")

Composition is done using the (.) operator which is part of the Control.Category module. Make sure to import this module and hide the default (.), id function from the Haskell Prelude.

Pure lenses.

type :-> f a = PureLens f a

Pure lens type specialized for pure accessor functions.

lens :: (f -> a) -> (a -> f -> f) -> f :-> a

Create a pure lens from a getter and a setter.

We expect the following law to hold:

 get l (set l a f) == a

Or, equivalently:

 set l (get l f) f == f

get :: (f :-> a) -> f -> a

Getter for a pure lens.

set :: (f :-> a) -> a -> f -> f

Setter for a pure lens.

modify :: (f :-> a) -> (a -> a) -> f -> f

Modifier for a pure lens.

Views using Applicative.

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 datatype that only reveals the age and city. This can be done by using a neat Applicative functor instance:

import Control.Applicative

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 datatype (which is itself a lens again), we can use the modify function to make Jan move to Amsterdam over exactly two years:

moveToAmsterdamOverTwoYears :: Person -> Person
moveToAmsterdamOverTwoYears = modify ageAndCity (\(a, _) -> (a+2, "Amsterdam"))

ghci> moveToAmsterdamOverTwoYears jan
Person "Jan" 73 True (Place "Amsterdam" "The Netherlands" "Europe")

newtype Lens arr f a

Abstract Lens datatype. The getter and setter functions work in some arrow. Arrows allow for effectful lenses, for example, lenses that might fail or use state.

Constructors

Lens (Point arr f a a)

Instances

Arrow arr => Iso arr (Lens arr f)

We can diverge Lenses using an isomorphism.

ArrowApply arr => Category (Lens arr)

Working with bijections and isomorphisms.

This package contains a bijection datatype that encodes bidirectional functions. Just like lenses, bijections can be composed using the Control.Category type class. Bijections 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 = Bij show read `iso` age

data Bijection arr a b

The bijections datatype, an arrow that works in two directions.

Constructors

Bij

Fields

fw :: a `arr` b

bw :: b `arr` a

Instances

Arrow arr => Iso arr (Bijection arr a)

We can diverge Bijections using an isomorphism.

Category arr => Category (Bijection arr)

Bijections as categories.

class Iso arr f where

The isomorphism type class is like a Functor but works in two directions.

Methods

iso :: Bijection arr a b -> f a `arr` f b

Instances

Arrow arr => Iso arr (Bijection arr a)

We can diverge Bijections using an isomorphism.

Arrow arr => Iso arr (Lens arr f)

We can diverge Lenses using an isomorphism.

for :: Arrow arr => (i `arr` o) -> Lens arr f o -> Point arr f i o

Derive labels using Template Haskell.

We can either derive labels with or without type signatures. In the case of multi-constructor datatypes some fields might not always be available and the derived labels will be partial. Partial labels are provided with an additional type context that forces them to be only usable using the functions from Data.Label.Maybe.

mkLabels :: [Name] -> Q [Dec]

Derive lenses including type signatures for all the record selectors for a collection of datatypes. The types will be polymorphic and can be used in an arbitrary context.

mkLabel :: Name -> Q [Dec]

Derive lenses including type signatures for all the record selectors in a single datatype. The types will be polymorphic and can be used in an arbitrary context.

mkLabelsWith :: (String -> String) -> [Name] -> Q [Dec]

Generate the label name from the record field name. For instance, drop 1 . dropWhile (/='_') creates a label val from a record Rec { rec_val :: X }.

mkLabelsMono :: [Name] -> Q [Dec]

Derive lenses including type signatures for all the record selectors in a datatype. The signatures will be concrete and can only be used in the appropriate context.

mkLabelsNoTypes :: [Name] -> Q [Dec]

Derive lenses without type signatures for all the record selectors in a datatype.