Haskell类型类速记

所以,我有一对类型类,我将在一起使用很多,我希望避免每次都指定这两个类型。基本上,而不是

:: (Ord a, Fractional a, Ord b, Fractional b, ... Ord z, Fractional z) =>

在所有类型规范的开头,我宁愿

:: (OrdFractional a, OrdFractional b, ... OrdFractional z)

所以,我最初的想法是如何做到这一点,只是声明一个新的typeclass

module Example where

class (Fractional a, Ord a) => OrdFractional a

example :: (OrdFractional a, OrdFractional b) => (a,b) -> (a,b) -> (a,b) -> Bool
example (x1,y1) (x2,y2) (x3,y3) = (x1/x2 < x2/x3) && (y1/y2 < y2/y3)

但这并不像我希望的那样自动工作:

% ghci
Prelude> :l Example.hs
Ok, modules loaded: Example.
Prelude Example> example (1::Float,3::Float) (2,2) (3,1)

<interactive>:1:0:
    No instance for (OrdFractional Float)
      arising from a use of `example' at <interactive>:1:0-39
    Possible fix:
      add an instance declaration for (OrdFractional Float)
    In the expression: example (1 :: Float, 3 :: Float) (2, 2) (3, 1)
    In the definition of `it':
        it = example (1 :: Float, 3 :: Float) (2, 2) (3, 1)

手动创建实例似乎是一种拖拽,因此,接下来,我想我可能会尝试自动创建实例:

module Example where

class OrdFractional a
instance (Fractional a, Ord a) => OrdFractional a

example :: (OrdFractional a, OrdFractional b) => (a,b) -> (a,b) -> (a,b) -> Bool
example (x1,y1) (x2,y2) (x3,y3) = (x1/x2 < x2/x3) && (y1/y2 < y2/y3)

但是编译器不喜欢这样:

ghc -c Example.hs

Example.hs:4:0:
    Illegal instance declaration for `OrdFractional a'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are type *variables*,
         and each type variable appears at most once in the instance head.
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `OrdFractional a'

有什么方法可以做到吗?