Dynamic dispatch mechanism of OOP
Existential types in conjunction with type classes can be used to emulate the dynamic dispatch mechanism of object oriented programming languages. To illustrate this concept I show how a classic example from object oriented programming can be encoded in Haskell.
class Shape_ a where perimeter :: a -> Double area :: a -> Double data Shape = forall a. Shape_ a => Shape a type Radius = Double type Side = Double data Circle = Circle Radius data Rectangle = Rectangle Side Side data Square = Square Side instance Shape_ Circle where perimeter (Circle r) = 2 * pi * r area (Circle r) = pi * r * r instance Shape_ Rectangle where perimeter (Rectangle x y) = 2*(x + y) area (Rectangle x y) = x * y instance Shape_ Square where perimeter (Square s) = 4*s area (Square s) = s*s instance Shape_ Shape where perimeter (Shape shape) = perimeter shape area (Shape shape) = area shape -- -- Smart constructor -- circle :: Radius -> Shape circle r = Shape (Circle r) rectangle :: Side -> Side -> Shape rectangle x y = Shape (Rectangle x y) square :: Side -> Shape square s = Shape (Square s) shapes :: [Shape] shapes = [circle 2.4, rectangle 3.1 4.4, square 2.1]
(You may see other Smart constructors for other purposes).
The type of the
parse function for this GADT is a good example to illustrate the concept of existential type.
Examples from the Essential Haskell Compiler project
See the documentation on EHC, each paper at the Version 4 part:
- Chapter 8 (EH4) of Atze Dijksta's Essential Haskell PhD thesis (most recent version). A detailed explanation. It explains also that existential types can be expressed in Haskell, but their use is restricted to data declarations, and the notation (using keyword
forall) may be confusing. In Essential Haskell, existential types can occur not only in data declarations, and a separate keyword
existsis used for their notation.
- Essential Haskell Compiler overview