Question

当我尝试使用ghci加载我的代码时，它给了我这个错误：

Asig1.hs:4:1:
    Non type-variable argument in the constraint: Num [c]
    (Use FlexibleContexts to permit this)
    When checking that ‘f’ has the inferred type
      f :: forall c (t :: * -> *).
           (Num c, Num [c], Foldable t) =>
           [c] -> [c] -> t [c] -> ([c], [c])

我不明白我做错了什么。这是我的代码：

module Asig1 where

f as ys x = (s,z)
       where
       ws = zipWith (*) as ys
       s = foldl (+) ws x
       z = s

Answer 1

因为您使用foldl可能是错误的方法。 input[type=range] { -webkit-appearance: none; /* Hides the slider so that custom slider can be made */ width: 100%; /* Specific width is required for Firefox. */ background: transparent; /* Otherwise white in Chrome */ } input[type=range]::-webkit-slider-thumb { -webkit-appearance: none; } input[type=range]:focus { outline: none; /* Removes the blue border. You should probably do some kind of focus styling for accessibility reasons though. */ } input[type=range]::-ms-track { width: 100%; cursor: pointer; /* Hides the slider so custom styles can be added */ background: transparent; border-color: transparent; color: transparent; }有签名：

foldl

所以你给它一个函数（这里是Foldable t => (b -> a -> b) -> b -> t a -> b）一个初始值和一系列值。您可以使用以下命令修复代码：

module Asig1 where

f as ys x = (s,z)
       where
       ws = zipWith (*) as ys
       s = foldl (+) x ws --instead of foldl (+) ws x
       z = s

您可以进一步改进代码，因为(+)不是必需的：

module Asig1 where

f as ys x = (s,s)
       where
       ws = zipWith (*) as ys
       s = foldl (+) x ws --instead of foldl (+) ws x

Haskell错误约束中的非类型变量参数：Num [E]

1 个答案: