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，我想读取数组累加器的长度作为函数的最终计算。是否有可能这样做或这是对 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。如果我们在此上下文中使用 id，则类型将是...

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)，returns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。出于某种原因，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，可以传入 id，如果您只想获取累加值，那么 a <- snd <$> listens id 应该可以。

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只能给你一个计算完成的累加器。

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

How do I access both the value and accumulator of a Writer monad within a 'do' block (PureScript)?

monads

writer-monad

purescript