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type Graph = Vector [Int]

data BfsState = BfsState { queue :: Seq [Int]
                         , visited :: Set Int } 

pop :: MaybeT (State BfsState) [Int]
pop = do
    (x :< xs) <- viewl <$> gets queue
    modify $ \y -> y { queue = xs }
    return x

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class Hashable a where
    hash :: Bits b => a -> b

instance Hashable Int where
    hash = id

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fib :: Int -> Int
fib = memo fib'
    where fib' 0 = 1
          fib' 1 = 1
          fib' n = fib' (n - 1) + fib' (n - 2)

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{-# LANGUAGE ViewPatterns #-}
module Memo where

import Data.Bits (testBit, setBit, finiteBitSize)

data Memo a b = Fork (Memo a b) b (Memo a b)
    deriving Show

type Bit = Bool
newtype Bits = Bits [Bit]

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{-# LANGUAGE ViewPatterns #-}
module Memo where

import Data.Bits (testBit, setBit, finiteBitSize)

data Memo a b = Fork (Memo a b) b (Memo a b)
    deriving Show

type Bit = Bool
newtype Bits = Bits [Bit]

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{-
    (x - xs)^2 + (y - ys)^2 + (z - zs)^2 = r^2
    x(t) = xr + t * xv
    y(t) = yr + t * yv
    z(t) = zr + t * zv

    (xr - xs + t * xv)^2 + (yr - ys + t * yv)^2 + (zr - zs + t * zv)^2 = r^2
    
    t^2(xv^2 + yv^2 + zv^2) + 2t(xv(xr - xs) + yv(yr - ys) + zv(zr - zs)) +
    (xr - xs)^2 + (yr - ys)^2 + (zr - zs)^2 - r^2 = 0

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nubRuns :: Eq a => [a] -> [a]
nubRuns []       = []
nubRuns (x : xs) = foldr (\x list -> if x == head list then list else x : list) [x] xs

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class Equal x y b | x y -> b
instance x ~ y => Equal x y True
instance Equal x y False

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class Equal x y b | x y -> b
instance Equal x x True
instance Not True b => Equal x y b

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class AList a
instance AList Nil
instance (A x, AList xs) => AList (Cons x xs)