重载函数签名haskell [英] Overloading function signatures haskell

问题描述

我在编译时收到以下错误消息:

I get the following error message when I compile:

重复的类型签名:
weightedMedian.hs:71:0-39:findVal :: [ValPair]-> Double-> Double
weightedMedian.hs:68:0-36:findVal :: [ValPair]-> Int-> Double

Duplicate type signature:
weightedMedian.hs:71:0-39: findVal :: [ValPair] -> Double -> Double
weightedMedian.hs:68:0-36: findVal :: [ValPair] -> Int -> Double

我的解决方案是使用findValI和findValD.但是，findValI只是将Int类型转换为Double并调用findValD.

My solution is to have findValI and findValD. However, findValI just converts the Int type to a Double and calls findValD.

我也无法对Num类型(Int，Double)进行模式匹配，因此我不能仅将类型签名更改为

Also I can't pattern match on types of Num (Int, Double) so I can't just change the type signature to

findVal :: [ValPair] -> Num -> Double   

在许多语言中，我不需要其他名称.为什么在Haskell中需要其他名称?这会很难添加到语言中吗?还是那里有龙?

In many languages I wouldn't need different names. Why do I need different names in Haskell? Would this be hard to add to the language? Or are there dragons there?

推荐答案

在Haskell中，按类型分类提供了特殊的多态性(和名称重载):

Ad-hoc polymorphism (and name overloading) are provided in Haskell by typeclasses:

class CanFindVal a where
          findVal :: [ValPair] -> a -> Double

instance CanFindVal Double where
     findVal xs d = ...

instance CanFindVal Int where
     findVal xs d = findVal xs (fromIntegral d :: Double)

请注意，在这种情况下，由于findVal确实"需要一个Double，所以我总是总是将它加倍，并且当我需要将其传递给int时，只需在fromIntegral处使用fromIntegral即可.呼叫网站.通常，当涉及的行为或逻辑实际上不同时(而不是混杂地)，就需要类型类.

Note that in this case, since findVal "really" needs a Double, I'd just always have it take a double, and when I needed to pass it an int, just use fromIntegral at the call site. You generally want typeclasses when there's actually different behavior or logic involved, rather than promiscuously.

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