README.md

cantor-pairing

This package implements a beefed-up version of Enum via GHC generics called Cantor which works for both finite and countably-infinite types.

Example

import GHC.Generics
import Cantor

data MyType = MyType {
    value1 :: [ Maybe Bool ]
  , value2 :: Integer
  } deriving (Generic,Cantor,Show)

example :: IO ()
example = do
  putStrLn "The first 5 elements of the enumeration are:"
  print $ take 5 xs
  where
    xs :: [ MyType ]
    xs = cantorEnumeration

This should work nicely even with simple inductive types:

Recursive example

data Tree a = Leaf | Branch (Tree a) a (Tree a) deriving (Generic,Cantor)

If your type is finite, you can specify this by deriving the Finite typeclass, which is a subclass of Cantor:

Finite example

data Color = Red | Green | Blue deriving (Generic,Cantor,Finite)

Mutually-recursive types

If you have mutually-recursive types, unfortunately you'll need to manually specify the cardinality for now, but you can still get the to/from encodings for free:

data Foo = FooNil | Foo Bool Bar deriving (Generic,Show)
data Bar = BarNil | Bar Bool Foo deriving (Generic,Show)

instance Cantor Foo where
  cardinality = Countable
instance Cantor Bar

Once you have a valid instance of Cantor a, you may lazily inspect all values of the type using cantorEnumeration :: [ a ] and convert a point to and from its integer encoding using toCantor :: Integer -> a and fromCantor :: a -> Integer.