不在范围内:输入构造函数或类“-"(以及反转列表时的其他错误) [英] Not in scope: type constructor or class ‘-’ (and other errors while reversing a list)
问题描述
我正在研究七周内的七种语言"一书，Haskell上线第一天的问题之一是"编写一个接受列表并反向返回相同列表的函数".
I'm working through the "Seven Languages In Seven Weeks" book, and one of the questions in the first day of Haskell is to "Write a function that takes a list and returns the same list in reverse".
我的第一个实现效果很好:
My first implementation worked fine:
reverseList :: [a] -> [a]
reverseList [] = []
reverseList [x] = [x]
reverseList (h:t) = reverseList(t) ++ [h]
然后我想到了另一种方法，我试图将其实现为:
I then thought of another approach, which I tried to implement as:
reverseList1 :: [a] -> [a]
reverseList1 [] = []
reverseList1 [x] = [x]
reverseList1 l = last(l) :: reverseList1(take(length(l)-1)(l))
(即-取最后一个元素，然后将颠倒的first-all-but-one元素添加到该元素).但是，这在尝试加载文件时给了我一个错误(实际上是三个):
(That is - take the last element, then append to it the reversed first-all-but-one elements). However, this gave me an error (actually, three) when I tried to load the file:
[1 of 1] Compiling Day1 ( day1.hs, interpreted )
day1.hs:35:49: error: Not in scope: type constructor or class ‘-’
|
35 | reverseList1 l = last(l) :: reverseList1(take(length(l)-1)(l))
| ^^^^^^^^^^^
day1.hs:35:49: error:
Illegal operator ‘-’ in type ‘length (l) - 1’
Use TypeOperators to allow operators in types
|
35 | reverseList1 l = last(l) :: reverseList1(take(length(l)-1)(l))
| ^^^^^^^^^^^
day1.hs:35:59: error:
Illegal type: ‘1’ Perhaps you intended to use DataKinds
|
35 | reverseList1 l = last(l) :: reverseList1(take(length(l)-1)(l))
| ^
Failed, no modules loaded.
搜索不在范围内"错误提示我正在尝试使用未导入的内容-我不认为需要明确导入减"，根据其他实验.
Googling for the "not in scope" error suggested that I'm trying to use something which hasn't been imported - I don't think that I need to explicitly import "subtract", based on other experiments.
我不明白第二个错误(使用TypeOperators允许类型中的运算符)-我从
I don't understand the second error (Use TypeOperators to allow operators in types) - I see from here that TypeOperators is a flag I can set on ghci, but I'm not trying to use an operator in a type - length(l)-1 is (to the best of my knowledge) an argument to take, not a type.
Google搜索也许您打算使用DataTypes"导致我在此处-我希望，我的代码不会像该示例那样破烂不堪，但是我可能同样一无所知！
Googling "perhaps you intended to use DataTypes" led me to here - I hope that my code won't be as ripped-apart as that example, but it's possible that I'm equally clueless!
基于最后一个示例此处，我也尝试过:
Based on the last example here, I also tried:
reverseList1 l = last(l) :: reverseList1(take(takeLength)(l))
where takeLength = length(l)-1
((从无法匹配类型'a'"开始出现(很长！)错误)和
(which gave a (very lengthy!) error starting with "Couldn't match type 'a'") and
reverseList1 l =
let takeLength = length(l)-1 in
in last(l) :: reverseList1(take(takeLength)(l))
哪个给了
day1.hs:40:5: error: parse error on input ‘in’
|
40 | in last(l) :: reverseList1(take(takeLength)(l))
| ^^
Failed, no modules loaded.
推荐答案
列表是使用:构造的. :: 表示一种类型，因此Haskell尝试将其读取为一种类型，因此会出现错误.
Lists are constructed with :. :: denotes a type, and so Haskell tries to read it as a type, hence the error.
更正后的代码是:
reverseList1 :: [a] -> [a]
reverseList1 [] = []
reverseList1 [x] = [x]
reverseList1 l = last l : reverseList1 (take (length l - 1) l)
请注意使代码更具可读性的括号！
Note the bracketing that makes the code more readable!
在第二次尝试中，您在 in 中写了两次，解析器感到困惑.
In your second attempt, you write in twice and the parser gets confused.
更正后的代码是:
reverseList1 l =
let takeLength = length l - 1
in last l : reverseList1 (take takeLength) l
现在，该错误实际上很有趣. TypeOperators 扩展名是一种(毫不奇怪)允许在类型声明中使用中缀运算符的扩展名.
Now, the error is actually quite interesting. The TypeOperators extension is one that allows (unsurprisingly) infix operators in type declarations.
在这里，编译器注意到-看起来像是一个infix运算符，并且鉴于它的类型符号推断您可能打算使用 TypeOperators 扩展名.当然，这里不是这样！
Here, the compiler noticed that - looked like an infix operator, and given that it was in a type signaures inferred that you probably meant to use the TypeOperators extension. Of course, that wasn't the case here!
对于 DataKinds ，这是一个相当高级的扩展，允许您放置数据(例如 1 ，"Hello" 等)在类型上.我不会在这里确切使用它，但是编译器提到它的原因是您在类型签名中使用了 1 ，该签名只能与 DataKinds 扩展名.
As for DataKinds, that is a fairly advanced extension that allows you to put data (like 1, "Hello" etc.) in types. I won't go into exactly what it's used for here, but the reason that the compiler mentioned it is that you used 1 in a type signature, which can only be used with the DataKinds extension.
这篇关于不在范围内:输入构造函数或类“-"(以及反转列表时的其他错误)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
