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evaluate :: ComonadApply f => f (f b -> b) -> f b
evaluate fs = fix $ (fs <@>) . duplicate

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pascal :: Z2 Int Int Integer
pascal = evaluate $ sheet (0,0) 0 $
  repeat 1 : repeat (1 : pascalRow)
  where pascalRow = repeat $ cell above + cell left
  
pascalLists :: [[Integer]]
pascalLists = map pascalList [0..]
   where
      pascalList n =
         map view .

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pascalLists :: [[Integer]]
pascalLists = map pascalList [0..]
   where
      pascalList n =
         map view .
         takeWhile ((>= 0) . row) .
         iterate (go $ above & right) .
         goto (0,n) $ pascal

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> take 7 pascalLists
[[1],
 [1,  1], 
 [1,  2,  1], 
 [1,  3,  3,  1], 
 [1,  4,  6,  4,  1], 
 [1,  5, 10, 10,  5,  1], 
 [1,  6, 15, 20, 15,  6,  1], 
 [1,  7, 21, 35, 35, 21,  7,  1]]

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module MultiFilters where

import Control.Applicative ((<$>))

(.:) :: (c -> d) -> (a -> b -> c) -> a -> b -> d
a .: b = (a .) . b

juxt :: [a -> b] -> a -> [b]
juxt = sequence

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{-# LANGUAGE GADTs            #-}
{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE PolyKinds        #-}
{-# LANGUAGE KindSignatures   #-}
{-# LANGUAGE TypeFamilies     #-}
{-# LANGUAGE TypeOperators    #-}
{-# LANGUAGE RankNTypes       #-}
{-# LANGUAGE ConstraintKinds  #-}
{-# LANGUAGE RebindableSyntax #-}

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{-# LANGUAGE GADTs          #-}
{-# LANGUAGE DataKinds      #-}
{-# LANGUAGE KindSignatures #-}

module Truffula where

data Path = LeftOf Path | RightOf Path | Root

data Tree (p :: Path) a where
   Tree :: { value  :: a

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-- a Wrapper is a pair of *something*, and if that something is an Int, an additional boolean
data Wrapper a =
    Wrapper { get  :: a
            , test :: (a ~ Int) => Bool }
 
-- We can construct a Wrapper from something of any type
makeWrapper :: a -> Wrapper a
makeWrapper x = Wrapper x (x == 0) -- Whoa! We're comparing it to zero even if it might not be an Int!
 
-- We can construct a Wrapper, as noted above, at various type indices…

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{-# LANGUAGE DataKinds  #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs      #-}
{-# LANGUAGE PolyKinds  #-}

-- How to use fix to implement polymorphic recursion!

module PolymorphicFixedpoints where

import Data.Function ( fix )

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{-# LANGUAGE RankNTypes    #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UnicodeSyntax #-}

module DeMorganCPS where

-- Products and sums with prettier type notation
data a ∧ b = Pair a b
data a ∨ b = InL a | InR b