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pop :: State (Seq Int, Set Int) (Maybe [Int])
pop = do
    (x :< xs) <- viewl <$> gets fst
    modify $ const xs *** id
    return x

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pop :: State (Seq Int, Set Int) (Maybe [Int])
pop = do
    (x :< xs) <- viewl <$> gets fst
    modify $ const xs *** id
    return x

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    No instance for (MonadState
                       (Seq (Maybe [Int]), Set Int)
                       (StateT (Seq Int, Set Int) Data.Functor.Identity.Identity))
      arising from a use of `gets'
    In the second argument of `(<$>)', namely `gets fst'
    In a stmt of a 'do' block: (x :< xs) <- viewl <$> gets fst
    In the expression:
      do { (x :< xs) <- viewl <$> gets fst;
           modify $ const xs *** id;

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

push :: [Int] ->  MaybeT (State BfsState) ()
push x = do q <- gets queue
            modify $ \y -> y { queue = q |> x }

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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