Modulegeneric-lens-2.2.2.0Haskell2010

Data.Generics.Sum.Subtype

Structural subtype relationships between sum types.

1 class

Packagegeneric-lens-2.2.2.0
Exports1
LanguageHaskell2010
LicenceBSD-3-Clause
SourceSubtype.hs

Prisms

1 declaration

Running example:

Example6 expressions

>>> :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 = Stringtype Age  = Intdog, cat, duck :: Animaldog = Dog (MkDog "Shep" 3)cat = Cat "Mog" 5duck = Duck 2dog4, cat4 :: FourLeggedAnimaldog4 = Dog4 (MkDog "Snowy" 4)cat4 = Cat4 "Garfield" 6:}

classclass AsSubtype sub sup where

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).

Methods

_Sub :: Prism' sup sub
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).
Example1 expression
```
>>> _Sub # dog4 :: AnimalDog (MkDog {name = "Snowy", age = 4})
```
Example1 expression
```
>>> cat ^? _Sub :: Maybe FourLeggedAnimalJust (Cat4 "Mog" 5)
```
Example1 expression
```
>>> duck ^? _Sub :: Maybe FourLeggedAnimalNothing
```
injectSub :: sub -> sup
Injects a subtype into a supertype (upcast).
projectSub :: sup -> Maybe sub
Projects a subtype from a supertype (downcast).

Instances4AsSubtype

AsSubtype Void aDefined in generic-lens-2.2.2.0 · Data.Generics.Sum.Subtype
Uncluttering type signatures (see Void)
Example1 expression
```
>>> :t +d _Sub @Int_Sub @Int  :: (AsSubtype Int sup, Choice p, Applicative f) =>     p Int (f Int) -> p sup (f sup)
```
AsSubtype a VoidDefined in generic-lens-2.2.2.0 · Data.Generics.Sum.Subtype
Uncluttering type signatures (see Void)
Example1 expression
```
>>> :t +d _Sub_Sub  :: (AsSubtype sub sup, Choice p, Applicative f) =>     p sub (f sub) -> p sup (f sup)
```
AsSubtype a aDefined in generic-lens-2.2.2.0 · Data.Generics.Sum.Subtype
Reflexive case
Example1 expression
```
>>> _Sub # dog :: AnimalDog (MkDog {name = "Shep", age = 3})
```
Context sub sup => AsSubtype sub supDefined in generic-lens-2.2.2.0 · Data.Generics.Sum.Subtype