Copyright	(C) 2020 Csongor Kiss
License	BSD3
Maintainer	Csongor Kiss <kiss.csongor.kiss@gmail.com>
Stability	experimental
Portability	non-portable
Safe Haskell	None
Language	Haskell2010

Data.Generics.Sum.Subtype

Contents

Prisms

Description

Structural subtype relationships between sum types.

Synopsis

class AsSubtype sub sup where
- _Sub :: Prism' sup sub
- injectSub :: sub -> sup
- projectSub :: sup -> Maybe sub

Prisms

Running example:

>>> :set -XTypeApplications
>>> :set -XDataKinds
>>> :set -XDeriveGeneric
>>> import GHC.Generics
>>> import Control.Lens
>>> :{
data Animal
  = Dog Dog
  | Cat Name Age
  | Duck Age
  deriving (Generic, Show)
data FourLeggedAnimal
  = Dog4 Dog
  | Cat4 Name Age
  deriving (Generic, Show)
data Dog = MkDog
  { name :: Name
  , age  :: Age
  }
  deriving (Generic, Show)
type Name = String
type Age  = Int
dog, cat, duck :: Animal
dog = Dog (MkDog "Shep" 3)
cat = Cat "Mog" 5
duck = Duck 2
dog4, cat4 :: FourLeggedAnimal
dog4 = Dog4 (MkDog "Snowy" 4)
cat4 = Cat4 "Garfield" 6
:}

class AsSubtype sub sup where Source #

Structural subtyping between sums. A sum Sub is a subtype of another sum Sup if a value of Sub can be given (modulo naming of constructors) whenever a value of Sup is expected. In the running example for instance, FourLeggedAnimal is a subtype of Animal since a value of the former can be given as a value of the latter (renaming Dog4 to Dog and Cat4 to Cat ).

Minimal complete definition

injectSub , projectSub | _Sub

Methods

_Sub :: Prism' sup sub Source #

A prism that captures structural subtyping. Allows a substructure to be injected (upcast) into a superstructure or a superstructure to be downcast into a substructure (which may fail).

>>> _Sub # dog4 :: Animal
Dog (MkDog {name = "Snowy", age = 4})

>>> cat ^? _Sub :: Maybe FourLeggedAnimal
Just (Cat4 "Mog" 5)

>>> duck ^? _Sub :: Maybe FourLeggedAnimal
Nothing

injectSub :: sub -> sup Source #

Injects a subtype into a supertype (upcast).

projectSub :: sup -> Maybe sub Source #

Projects a subtype from a supertype (downcast).

Instances

Instances details

AsSubtype a Void Source #	See Note [Uncluttering type signatures] _Sub :: (AsSubtype sub sup, Data.Profunctor.Choice.Choice p, Applicative f) => p sub (f sub) -> p sup (f sup)
Instance details Defined in Data.Generics.Sum.Subtype Methods _Sub :: Prism' Void a Source # injectSub :: a -> Void Source # projectSub :: Void -> Maybe a Source #
AsSubtype a a Source #	Reflexive case >>> _Sub # dog :: Animal Dog (MkDog {name = Shep , age = 3})
Instance details Defined in Data.Generics.Sum.Subtype Methods _Sub :: Prism' a a Source # injectSub :: a -> a Source # projectSub :: a -> Maybe a Source #
Context sub sup => AsSubtype sub sup Source #
Instance details Defined in Data.Generics.Sum.Subtype Methods _Sub :: Prism' sup sub Source # injectSub :: sub -> sup Source # projectSub :: sup -> Maybe sub Source #
AsSubtype Void a Source #	See Note [Uncluttering type signatures] >>> :t +d _Sub `Int _Sub` Int :: (AsSubtype Int sup, Choice p, Applicative f) => p Int (f Int) -> p sup (f sup)
Instance details Defined in Data.Generics.Sum.Subtype Methods _Sub :: Prism' a Void Source # injectSub :: Void -> a Source # projectSub :: a -> Maybe Void Source #