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Bence's questions

Martin Hope
Bence
Asked: 2024-11-21 04:24:30 +0800 CST
  • 10

我是 Haskell 的新手，有一项大学家庭作业，编译后发现一行只有一个分号。我很好奇，这是否是预期的行为，如果是，为什么？

（过去几天我编写了一些有趣的代码行，但它们并没有按预期工作，每一个都是一个小讲座，例如，--|不是注释掉的保护，而是未定义的运算符等。）

我唯一能用的版本是（完整代码如下）：

   case input of {
       ... -- some other code
       ('0':xs)     -> [(next Zero, rest)] where (next, rest) = readAll xs
       ;
       ... -- some other code
   }

我想知道是否有可能在没有由单个分号组成的行的情况下做到这一点，以及为什么下面的例子不起作用。

我试过例如

       ('0':xs)     -> [(next Zero, rest)] where (next, rest) = readAll xs;

       ('0':xs)     -> [(next Zero, rest)] where (next, rest) = (readAll xs);

       ('0':xs)     -> [(next Zero, rest)]
          where
             (next, rest) = (readAll xs);

但它们都导致了同一种错误：

file.hs:19:8: error: parse error on input ‘(’
   |
19 |        ('1':xs)     -> [(next One , rest)] where (next, rest) = readAll xs
   |        ^

可运行的代码片段：

data BinaryDigit = Zero | One
data BinaryNumber = L BinaryDigit -- L: last digit
                  | E BinaryDigit BinaryNumber -- E: intermediate digit

instance Show BinaryNumber where
   show (L Zero) = "0"
   show (L One ) = "1"
   show (E digit next) = show (L digit) ++ show next

instance Read BinaryNumber where
     -- idea:
     --   next item must be known to decide whether E or L constructor to use
     --   create function that will be called on the next element and returns the right constructor,
     --   with only one parameter, the current digit value, unset
   readsPrec _ input = case input of {
       ('0':'0':xs) -> readsPrec 0 ('0':xs);
       ('0':'1':xs) -> readsPrec 0 ('1':xs);
       ('0':xs)     -> [(next Zero, rest)] where (next, rest) = readAll xs
       ;
       ('1':xs)     -> [(next One , rest)] where (next, rest) = readAll xs
       ;
       otherwise    -> [];
   } where
     readAll :: String -> (BinaryDigit -> BinaryNumber, String)
     readAll ('0':xs) = ((`E` (next Zero)), rest) where (next, rest) = readAll xs
     readAll ('1':xs) = ((`E` (next One )), rest) where (next, rest) = readAll xs
     readAll str      = (L, str)

main = putStrLn $ show (read "110" :: BinaryNumber)
haskell