解析模式错误 [英] Parse error on pattern

问题描述

试图实现一个函数，它将返回一个int列表，表示每个double列表的顺序，例如：

 订单[[1.0,2.0,3.0]，[3.0,2.0,1.0]] 
> [[0，1，2]，[2，1，0]] 
  

与我的模式匹配出于某种原因：

  import Data.List 
 
  - 返回列表每个双打名单的排序
 orderings :: [[Double]]  - > [Int] 
 orderings [] = [] 
排序x：xs =（排序x）:(排序xs）
 
排序:: [Double]  - > [Int] 
排序xs = [i | （i，_）< -  sorted]其中
 sorted = sortBy（\（i1，e1）（i2，e2） - > compare e1 e2）$ zip [0 ..] xs 
  

错误是：



 模式中的分析错误：排序
  

看不到我生命中的错误！ / p>


解决方案

还有两个问题（除了 x：xs ）：


1. orderings 类型是错误的;我怀疑它应该是 [[Double]] - > [[Int]]




2. x code>排序;我怀疑它应该是 xs

：

  import Data.List 
 
  - 返回每个双打列表的排序列表
排序:: [[Double]]  - > [[Int]]  - 更改类型
排序[] = [] 
排序（x：xs）=（排序x）：排序xs 
 
排序:: [Double]  - > [Int] 
排序xs = [i | （i，_）< -  sorted]其中
 sorted = sortBy（\（i1，e1）（i2，e2） - > compare e1 e2）$ zip [0 ..] xs  -  xs not X！ 
  

Trying to implement a function that will return a list of ints the represent an ordering of each list of doubles, e.g.:

orderings [ [1.0, 2.0, 3.0], [3.0, 2.0, 1.0] ]
> [ [0, 1, 2], [2, 1, 0] ]

However, having trouble with my pattern matching for some reason:

import Data.List

-- Return a list of orderings for each list of doubles
orderings:: [[Double]] -> [Int]
orderings [] = []
orderings x:xs = (ordering x):(orderings xs)

ordering:: [Double] -> [Int]
ordering xs = [i | (i, _) <- sorted] where
    sorted = sortBy (\(i1, e1) (i2,e2) -> compare e1 e2) $ zip [0..] xs

Error is:

Parse error in pattern: orderings

Can't see the error for the life of me!

解决方案

Two more problems (in addition to the missing parentheses around x:xs):

1. the type of orderings is wrong; I suspect it should be [[Double]] -> [[Int]]

2. x is not in scope in ordering; I suspect it should be xs

Here's the corrected code:

import Data.List

-- Return a list of orderings for each list of doubles
orderings:: [[Double]] -> [[Int]]  -- changed type
orderings [] = []
orderings (x:xs) = (ordering x):(orderings xs)

ordering:: [Double] -> [Int]
ordering xs = [i | (i, _) <- sorted] where
    sorted = sortBy (\(i1, e1) (i2,e2) -> compare e1 e2) $ zip [0..] xs -- xs not x!

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