Writerモナドの整理

Writer.hs

import Control.Monad.Writer

-- do構文でWriter
logCalcD :: Writer [String] Int
logCalcD = do
  tell ["Process start"]
  x <- writer (1, ["Initial value: " ++ " 1"])
  y <- writer (x * 2, ["Multiply by two"])
  tell ["Process end"]
  return y

-- 明示的にbind
-- (>>=)の左辺はWriter, 右辺の引数は前のWriterから得られた計算値(ここではInt型)
-- Writer w aの型wはモノイドでなければならない
logCalcB :: Writer [String] Int
logCalcB = tell ["Process Start"]
             >>= \_ -> writer (1, ["Initial value: " ++ "1"])
             >>= \x -> writer (x * 2, ["Multiply by two"])
             >>= \y -> writer (y, ["Process end"])