example of church encoding

encodings.hs

-- see https://docs.google.com/presentation/d/1f-avqUksUCO8vvwn1KQNu-LIVwqNc1EbQz1l_stTD8o/edit#slide=id.gf6104008a_0_810
{-# LANGUAGE RankNTypes #-}

type PersonLambdas = forall a. (String -> Int -> a) -> (String -> Int -> Double -> a) -> a

child :: String -> Int -> PersonLambdas
child s i = \childL adultL -> childL s i

adult :: String -> Int -> Double -> PersonLambdas
adult s i d = \_ adultL -> adultL s i d

name :: PersonLambdas -> String
name lam = let
  childL = \s _ -> s
  adultL = \s _ _ -> s
  in lam childL adultL

age :: PersonLambdas -> Int
age lam = let
  childL = \_ a -> a
  adultL = \_ a _ -> a
  in lam childL adultL

netWorth :: PersonLambdas -> Double
netWorth lam = let
  childL = \_ _ -> 0.0
  adultL = \_ _ nw -> nw
  in lam childL adultL

main = do
  let bobby = child "Bobby" 5
  print $ age bobby
  print $ name bobby
  let charles = adult "Charles" 24 (1.0/0.0)
  print $ netWorth charles
  print $ netWorth bobby

output.txt

5
"Bobby"
Infinity
0.0