为什么我不能在 ghci 中定义新类型？

发布于 2024-09-06 19:51:32 字数 241 浏览 8 评论 0原文

当我尝试定义新类型时，在 ghci 中出现错误：

Prelude> data Point = Pt Int Int
<interactive>:1:0: parse error on input `data'
Prelude> let data Point = Pt Int Int
<interactive>:1:4: parse error on input `data'

我做错了什么？

原文

I get an error in ghci when I try to define a new type:

Prelude> data Point = Pt Int Int
<interactive>:1:0: parse error on input `data'
Prelude> let data Point = Pt Int Int
<interactive>:1:4: parse error on input `data'

What am I doing wrong?

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大姐，你呐 2024-09-13 19:51:32

titaniumdecoy，我记得当我了解到经常提到的一点时，在 GHCi 中编写诸如 'let square x = x * x' 之类的东西就像编写 let a = f b 在 IO monad 中使用 do 表示法——在这种例子中说：

palindromize :: IO ()
palindromize = do
  a <- readFile "foo.txt"
  let b = reverse a
  putStrLn (a ++ b)

同样，当你在 GHCi 中重新定义一个表达式时，它有点像就像使用 do 表示法执行以下操作一样，这是完全合法的：

makeMess :: IO ()
makeMess = do
   a <- readFile "foo.txt"
   let b = reverse a
   putStrLn (a ++ b)
   let b = a
   putStrLn (a ++ b)

没有人会在这样的序列中间声明数据类型，但会在模块的其他地方执行此操作。我可能猜到存在某种理论上的反对意见，但唐·S. 的评论表明不存在这种反对意见。

titaniumdecoy, I remember being helped with this sort of GHCi mystery when I learned the frequently made point that writing things like 'let square x = x * x' inside the GHCi is like writing let a = f b with do notation in the IO monad -- say in this sort of example:

palindromize :: IO ()
palindromize = do
  a <- readFile "foo.txt"
  let b = reverse a
  putStrLn (a ++ b)

Similarly, when you redefine an expression in the GHCi, it's sort of like doing the following in do notation, which is perfectly legitimate:

makeMess :: IO ()
makeMess = do
   a <- readFile "foo.txt"
   let b = reverse a
   putStrLn (a ++ b)
   let b = a
   putStrLn (a ++ b)

No one would declare a data type in the middle of such a sequence, but would do it elsewhere in the module. I might have guessed that there was some sort of theoretical objection, but Don S.'s remark suggests there isn't one.

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