Question

因此函数即时编写应该采用一些点（punkte）和奖励点，然后将其与punkte列表进行比较并返回相应的等级（通知列表）。

我对haskell相当陌生并且不善于解释错误。

我经常得到无限类型错误我真的不知道为什么。你能解释一下吗？也许给出一个解决方案？

功能：

import Data.Char -- i know i don't need this so far
import Data.List
import Data.Maybe

punkte = [50,54..86] 
noten = [4.0,3.7,3.3,3.0,2.7,2.3,2.0,1.7,1.3,1.0]

berechneNote p bp
    | bp > 20 = error "too many bonuspoints"
    | bp < 0 = error "you can't give negative points"
    | p > 100 = error "too many points"
    | p < 0 = error "you can't give negative points"
    | otherwise = berechneNote2 (p+bp) punkte

-- this constructes an infinite type aparently ?
berechneNote2 p (x:xs)
    | p == x = noten !! (fromJust (elemIndex x p))
    | p > x = berechneNote2 p xs
    | p < x = noten !! (fromJust (elemIndex x p))

这就是我得到的错误

blatt1.hs:17:48: error:
• Occurs check: cannot construct the infinite type: a ~ [a]
• In the second argument of ‘elemIndex’, namely ‘p’
  In the first argument of ‘fromJust’, namely ‘(elemIndex x p)’
  In the second argument of ‘(!!)’, namely
    ‘(fromJust (elemIndex x p))’
• Relevant bindings include
    xs :: [a] (bound at blatt1.hs:16:20)
    x :: a (bound at blatt1.hs:16:18)
    p :: a (bound at blatt1.hs:16:15)
    berechneNote2 :: a -> [a] -> Double (bound at blatt1.hs:16:1)

Answer 1

| p == x = noten !! (fromJust (elemIndex x p))

来自p == x，p和x属于同一类型。我们将此命名为a。

来自elemIndex x p，p必须是列表类型，例如[b]。此外，x必须是p的潜在元素，因此必须包含b类型。

因此，我们得到a ~ [b] ~ [a]，这是无意义的：列表不能包含相同列表类型的元素。

此外，我们强烈建议为每个顶级定义提供预期类型。通过这种方式，GHC将产生更好的类型错误，指出我们打算定义的类型与代码产生的类型之间的差异。

Haskell非类型变量参数错误

1 个答案: