14_For_a_Few_Monads_More.md

Writer Monad

newtype Writer w a = Writer { runWriter :: (a, w) }

instance (Monoid w) => Monad (Writer w) where
    return x = Writer (x, mempty)
    (Writer (x, v)) >>= f = let (Writer (y, v')) = f x 
                              in Writer (y, v `mappend` v')

基本的には、値とモノイド値のペアをラップした値が Writer
値を受け取りWriter値を返す関数をbindできる
bind演算の結果は次のようなWriter値となる
- (計算値, 元のWriterのモノイド値へ計算で生じたモノイド値をmappendしたもの)
Writer は値コンストラクタをエクスポートしていないので、writer 関数などを使う

> let hoge = return 10 :: Writer [String] Int
> :t hoge
hoge :: Writer [String] Int
> runWriter hoge
(10,[])
> let a = writer (12470, ["hoge"])
> :t a
a :: (MonadWriter [[Char]] m, Num a) => m a
> runWriter a
(12470,["hoge"])

以下のようにしてログ機構を実現できる。tellを使うとモノイド値へmappendしたい値を渡せる。

import Control.Monad.Writer

logNumber :: Int -> Writer [String] Int 
logNumber x = writer (x, ["Got number: " ++ show x])

multWithLog :: Writer [String] Int
multWithLog = do
    a <- logNumber 3
    tell ["Between A and B"]
    b <- logNumber 5
    return (a + b)

-- on ghci
> runWriter multWithLog
(8, ["Got number: 3","Between A and B","Got number: 5"])

差分リスト

newtype DiffList a = DiffList { getDiffList :: [a] -> [a] }

toDiffList :: [a] -> DiffList a
toDiffList xs = DiffList (xs++)

fromDiffList :: DiffList a -> [a]
fromDiffList (DiffList f) = f []

instance Monoid (DiffList a) where
    mempty = DiffList (\xs -> [] ++ xs)
    (DiffList f) `mappend` (DiffList g) = DiffList (\xs -> f (g xs))

> -- ghci
> fromDiffList $ toDiffList [1,2,3] `mappend` toDiffList [5,6,7]
[1,2,3,5,6,7]

Reader Monad

モナドとしての関数は「値がまだ手元になく、何かに適用されると値が出てくる」という文脈を持ちます

instance Monad ((->) r) where
    return x = \_ -> x
    h >>= f = \w -> f (h w) w

53ningen/14_For_a_Few_Monads_More.md

Writer Monad

差分リスト

Reader Monad