Haskell非疲惫

Question

我想知道是否有人可以帮我识别出非详尽的haskell代码部分？我无法看到列表末尾的基本情况是如何得到满足的。

非常感谢

强尼

type Rule
  = (Char, String)

type Rules
  = [Rule]

type System
  = (Float, String, Rules)

cross, triangle, arrowHead, peanoGosper,
  dragon, snowflake, tree, bush :: System

type Vertex
  = (Float, Float)

type TurtleState
  = (Vertex, Float)

type Stack
  = [TurtleState]

type ColouredLine
  = (Vertex, Vertex, Colour)


trace :: String -> Float -> Colour -> [ColouredLine]

trace rules angle colour = traceState rules ((0,0),90) []

  where

    traceState :: String -> TurtleState -> Stack -> [ColouredLine]
    traceState [] _ _ = []
    traceState (x:xs) t (y:ys)
      |x == '[' = traceState xs t (t : (y:ys))
      |x == ']' = traceState xs t ys
      |x == 'F' = biggieSmalls : traceState xs t (nextY:ys)
      |otherwise = traceState xs angledY (angledY:ys)
        where
          biggieSmalls = (fst(t),fst(nextY),colour)
          nextY = move 'F' t angle
          angledY = move x t angle

Answer 1

正如Benjamin Hodgson所指出的，你还没有匹配rules字符串非空（(_ : _)而不是[]）而Stack的情况。 } 是空的。这种情况会立即出现，因为初始堆栈trace传递给traceState实际上是[]。

要修正警告/例外，您需要将traceState的第二个等式重写为traceState (x : xs) t y，或者编写第三个等式tracestate (x : xs) t []。使用前一种解决方案，您可以使用case y of在y上进行模式匹配。无论哪种方式，您都需要确定如何处理列表为空的情况。

问题实际上会出现在traceState "]" t []之类的调用中，即trace的第一个参数是以＆＃34;]＆＃34;或＆＃34; F＆＃34;。看起来这是一个无效的输入。如果是这种情况，您可能希望重写trace和traceState以返回Maybe [ColouredLine]，或使用Either来提供错误消息。