haskell - Haskell 向量类型声明

Question

我正在编写一个要传递给的比较器，sortBy但我无法正确声明类型。输入是两个Data.Vector，每个包含两个数字。

-- Comparator to sort a list of individuals by increasing order of fit-0 
--      and for individuals with equal fit-0, with increasing order of fit-1
indCmp :: (Ord a, Num a, Vector a)  => a -> a -> Ordering
indCmp x y
    | (x ! 0) < (y ! 0) = LT
    | (x ! 0) > (y ! 0) = GT
    | (x ! 1) < (y ! 1) = LT -- Can assume (x ! 0) == (y ! 0) here and beneath
    | (x ! 1) > (y ! 1) = GT
    | (x ! 1) == (y ! 1) = EQ

GHCI 抱怨：

预期有约束，但“向量 a”有种类“*”

Answer 1

Vector是数据类型，而不是类，所以你的函数类型应该是

indCmp :: (Ord a, Num a)  => Vector a -> Vector a -> Ordering

当我改变它时，它为我编译。