algorithm - 你会如何在 Haskell 中表达它？

Question

你会使用 if/else 在 Haskell 中编写这个算法吗？没有他们，有没有办法表达它？很难从中间提取出有意义的函数。这只是机器学习系统的输出。

我正在实现用于将 html 内容段分类为内容或样板的算法 here .这具有已经硬编码的权重。

curr_linkDensity <= 0.333333
| prev_linkDensity <= 0.555556
| | curr_numWords <= 16
| | | next_numWords <= 15
| | | | prev_numWords <= 4: BOILERPLATE
| | | | prev_numWords > 4: CONTENT
| | | next_numWords > 15: CONTENT
| | curr_numWords > 16: CONTENT
| prev_linkDensity > 0.555556
| | curr_numWords <= 40
| | | next_numWords <= 17: BOILERPLATE
| | | next_numWords > 17: CONTENT
| | curr_numWords > 40: CONTENT
curr_linkDensity > 0.333333: BOILERPLATE

Answer 1

不是手动简化逻辑(假设您可能会自动生成此代码)，我认为使用 MultiWayIf非常干净直接。

{-# LANGUAGE MultiWayIf #-}

data Stats = Stats {
    curr_linkDensity :: Double,
    prev_linkDensity :: Double,
    ...
}

data Classification = Content | Boilerplate

classify :: Stats -> Classification
classify s = if
    | curr_linkDensity s <= 0.333333 -> if
      | prev_linkDensity s <= 0.555556 -> if
        | curr_numWords s <= 16 -> if
          | next_numWords s <= 15 -> if
            | prev_numWords s <= 4 -> Boilerplate
            | prev_numWords s > 4 -> Content
          | next_numWords s > 16 -> Content
      ...

等等。

但是，由于这是结构化的——只是一个带有比较的 if/else 树，还可以考虑创建一个决策树数据结构并为其编写一个解释器。这将允许您进行转换、操作和检查。也许它会给你买东西；为您的规范定义微型语言可能非常有益。

data DecisionTree i o 
    = Comparison (i -> Double) Double (DecisionTree i o) (DecisionTree i o)
    | Leaf o

runDecisionTree :: DecisionTree i o -> i -> o
runDecisionTree (Comparison f v ifLess ifGreater) i
    | f i <= v  = runDecisionTree ifLess i
    | otherwise = runDecisionTree ifGreater i
runDecisionTree (Leaf o) = o

-- DecisionTree is an encoding of a function, and you can write
-- Functor, Applicative, and Monad instances!

然后

 classifier :: DecisionTree Stats Classification
 classifier =
     Comparison curr_linkDensity 0.333333
       (Comparison prev_linkDensity 0.555556
         (Comparison curr_numWords 16
           (Comparison next_numWords 15
             (Comparison prev_numWords 4
               (Leaf Boilerplate)
               (Leaf Content))
             (Leaf Content)
           ...