时间戳

首页 > 解决方案 > Haskell 错误：无法将类型“a”与“Double”匹配

haskell - Haskell 错误：无法将类型“a”与“Double”匹配

问题描述

我正在编写一个 Haskell 代码来查找 k 个最近邻居的列表，按照距离顺序，根据度量 d 到点列表 xs 中的点 p。我的代码如下

import Data.List
import Data.Function

type Point a = (a,a)
type Metric a = (Point a) -> (Point a) -> Double

neighbours :: Int -> Metric a -> Point a -> [Point a] -> [Point a]
neighbours k calDistance p [] = []
neighbours k calDistance p ps
 | k < 0     = error "k cannot be negative"
 | otherwise = take k (neighbours' (sortBy (compare `on` snd) [ (poi,dis) | (poi,dis) <- points p ps]))

neighbours' :: [(Point a, Double)] -> [Point a]
neighbours' xs = [ x | (x,y)<-xs]

points :: Point a -> [Point a] -> [(Point a, Double)]
points _ [] = []
points p1 (p2:px) = (p2,(calDistance p1 p2)): points p1 px

calDistance :: Metric Double
calDistance (x1,y1) (x2,y2) = sqrt((x1-x2)**2 + (y1-y2)**2)

错误是

    • Couldn't match type ‘a’ with ‘Double’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          points :: forall a. Point a -> [Point a] -> [(Point a, Double)]
        at A4.hs:27:1-53
      Expected type: Point Double
        Actual type: Point a
    • In the first argument of ‘calDistance’, namely ‘p1’
      In the expression: (calDistance p1 p2)
      In the first argument of ‘(:)’, namely ‘(p2, (calDistance p1 p2))’
    • Relevant bindings include
        px :: [Point a] (bound at A4.hs:29:15)
        p2 :: Point a (bound at A4.hs:29:12)
        p1 :: Point a (bound at A4.hs:29:8)
        points :: Point a -> [Point a] -> [(Point a, Double)]
          (bound at A4.hs:28:1)
   |
29 | points p1 (p2:px) = (p2,(calDistance p1 p2)): points p1 px
   |

但是，如果我将“Metric Double”更改为“Double a”，也会出现编译错误：无法将预期类型“Double”与实际类型“a”匹配。请问有人可以告诉我如何解决吗？

标签： haskell

解决方案

见在执行

points :: Point a -> [Point a] -> [(Point a, Double)]

你打电话

calDistance :: Metric Double

这意味着，这a必须是Double，如果有人points以类型的值Point Int作为第一个参数调用会发生什么？应该如何calDistance处理，因为它只能处理Point Double?

该错误告诉您，这calDistance是使用 type 的值调用的Point a，其中a可以是任何类型。calDistance这与预期的 aPoint Double作为它的第一个参数不匹配。

推荐阅读