根据模式排列在 Haskell 中获取 <<loop>> 异常

Question

谁能解释一下为什么我在这段代码中出现了一个循环？

module Main where

import Data.List.Split
import Data.Maybe
import Text.Read

main :: IO ()
main = print (snd (toInmetDate "01/02/2012"))

type P a = (Bool, a)

readP :: (Read a) => String -> P a
readP text
  | isJust value    = (True, fromJust value)
  | isNothing value = (False, read "0")
  where value = readMaybe text

data InmetDate = InmetDate {dia :: P Int, mes :: P Int, ano :: P Integer}
  deriving (Show, Read)

toInmetDate :: String -> P InmetDate
toInmetDate x = if length l == 3
  then  (True, InmetDate (readP ds) (readP ms) (readP as))
  else  (False, InmetDate (False, 0) (False, 0) (False, 0))
  where (l,ds:ms:as:_) = (splitOn "/" x, l ++ replicate 20 "NULL")

我会说，当我制作时：

  where (l,ds:ms:as:_) = (splitOn "/" x, l ++ replicate 20 "NULL")

等于：

  where (ds:ms:as:_) = l ++ replicate 20 "NULL"
        l = splitOn "/" x

代码完美运行。

Answer 1

在您的原始文件中，l 的值取决于根据 l 递归定义的元组。在更改中，l 仅取决于 splitOn "/" x 的值。