免费（作家 w）作家 w

特定

data Writer w a = Writer w a

我们有

data Free (Writer w) a
     = Pure a
     | Free (Writer w (Free (Writer w) a))

这是同构的

data ProgLog w a
     = Done a
     | After w (ProgLog w a)

或者，等效地（如果你承诺首先评估日志），Writer [w] a。