如何在“do”块（PureScript）中访问 Writer monad 的值和累加器？

Question

我现在正在学习 Writer monad，我不确定在 do 块中读取 monad 的值和累加器是否正确。例如，在下面的 coltzSeq 函数中，我想读取 Array 累加器的长度作为函数的最终计算。是否可以这样做或这是对 Writer 的不正确使用？显然，我可以让调用者读取最终数组的长度，或者我可以使用 State monad，但这只是我的练习。

module Main where

import Prelude
import Data.Tuple
import Control.Monad.Writer
import Math (remainder, (%))
import Data.Int (toNumber, fromNumber)
import Control.Monad.Eff.Console (logShow)

coltz :: Number -> Number
coltz n = case (n % 2.0 == 0.0) of
            true  -> n / 2.0
            false -> 3.0 * n + 1.0

coltzW :: Number -> Writer (Array Number) Number
coltzW n = do
  tell [n]
  pure $ coltz n

-- Computes a coltz sequence and counts how many
-- steps it took to compute
coltzSeq :: Number -> Writer (Array Number) Int
coltzSeq n = do

  -- Can read the value in the writer
  -- but not the writer's internal state
  v <- (coltzW n)
  let l = 1

  -- Can read the value and the internal
  -- state, but it's not bound to the monad's context.
  -- let a = runWriter (coltzW n)
  -- let v = fst a
  -- let l = length (snd a)
  case (v) of
    1.0 -> pure $ l
    _ -> to1 v

编辑： 我尝试了 gb. 的建议并尝试使用类型为 (Monoid w, Monad m) => forall w m a b. MonadWriter w m => (w -> b) -> m a -> m (Tuple a b) 的 listens 函数。如果我们在这种情况下使用id，类型将是...

MonadWriter w m => (w -> b) -> m a -> m (Tuple a b)

w = Array Number
m = WriterT (Array Number) Identity (alias: Writer (Array Number) )
b = Array Number
a = Number

(Array Number -> Array Number) ->
Writer (Array Number) Number ->
Writer (Array Number) (Tuple Number (Array Number))

所以listens id 接受Writer (Array Number) Number)，并返回一个值为当前 Writer 状态的 Writer（因为我们使用了id）。但是，我尝试和使用listens的所有方式都不断出现类型错误@

to1 :: Number -> (Writer (Array Number)) Int
to1 n = do
  v <- (coltzW n)
  -- a <- snd <$> listens id
  -- let l = snd <$> (listens id (execWriter (coltzW n)))
  -- let l = execWriter (listens id (coltzW n))

  -- Seems like this one should work to get Array Number
  -- let l = snd <$> (listens id (coltzW n))

  case (v) of
    1.0 -> pure 1
    _   -> to1 v

编辑2： 我想出了我需要做什么。由于某种原因，我需要在使用listens 时添加类型注释。

lengthOfSeq :: Writer (Array Number) Int -> Writer (Array Number) Int
lengthOfSeq c = do
  -- Without type annotation, I get this error...
  -- No type class instance was found for
  --                                                                             
  -- Control.Monad.Writer.Class.MonadWriter (Array t0)                       
  --                                        (WriterT (Array Number) Identity)
  --                                                                             
  -- The instance head contains unknown type variables. Consider adding a type annotation.
  Tuple a w <- (listens id c :: Writer (Array Number) (Tuple Int (Array Number)))
  pure $ length w

to1 :: Number -> (Writer (Array Number)) Int
to1 n = lengthOfSeq $ seq n
  where
  seq n = do
    v <- coltzW n
    case (v) of
      1.0 -> do
        pure 1
      _   -> seq v

Answer 1

我认为 listens 是您正在寻找的功能：https://pursuit.purescript.org/packages/purescript-transformers/1.0.0/docs/Control.Monad.Writer#v:listens，如果我理解正确的话，您正在尝试做什么。

如果您对转换值不感兴趣，可以传递id，如果您只想获取累积值，那么a <- snd <$> listens id 应该可以解决问题。

Answer 2

您无法访问当前累加器。 Writer 是只写的。如果您需要电流累加器，请使用 State 而不是 Writer。 listen 只能给你一个完成计算的累加器。