haskell - 无法推导出 (Eq a)，将 Eq 添加到 typeclass

Question

我对 Haskell 很陌生，在尝试编译 Frag 时遇到了这个错误。

src/AFRPVectorSpace.hs:51:25:
    Could not deduce (Eq a) arising from a use of `/='
    from the context (VectorSpace v a)
      bound by the class declaration for `VectorSpace'
      at src/AFRPVectorSpace.hs:(32,1)-(53,23)
    Possible fix:
      add (Eq a) to the context of
        the class declaration for `VectorSpace'
    In the expression: nv /= 0
    In the expression:
      if nv /= 0 then v ^/ nv else error "normalize: zero vector"
    In an equation for `normalize':
        normalize v
          = if nv /= 0 then v ^/ nv else error "normalize: zero vector"
          where
              nv = norm v

相关代码:

class Floating a => VectorSpace v a | v -> a where
    zeroVector   :: v
    (*^)         :: a -> v -> v
    (^/)         :: v -> a -> v
    negateVector :: v -> v
    (^+^)        :: v -> v -> v
    (^-^)        :: v -> v -> v
    dot          :: v -> v -> a
    norm     :: v -> a
    normalize    :: v -> v

    v ^/ a = (1/a) *^ v

    negateVector v = (-1) *^ v

    v1 ^-^ _ = v1 ^+^ v1 -- (negateVector v2)

    norm v = sqrt (v `dot` v)

    normalize v = if nv /= 0 then v ^/ nv else error "normalize: zero vector"
        where
        nv = norm v

我的第一个猜测是我需要添加一个 Deriving Eq 或类似的东西，但我不确定我到底需要做什么。

Answer 1

如果您想在默认实现中将 class (Eq a,Floating a) => VectorSpace v a | v -> a 用于 /=，我猜您需要有 a。

第二种选择是从类中删除 normalize 并使其成为普通函数。

第三种选择是将约束添加到 normalize 的类型，使其成为 Eq a => v -> v 。