将monad限制为类型类

M a 因此 a 满足类型类约束?

我正在翻译这本书 probabilistic modeling example F# 到 Haskell support 因为它会改变 data Distribution a data (Ord a) => Distribution a . 通过此更改,我得到以下错误:

...probabilisticModeling.hs:42:13:
    Could not deduce (Ord a) from the context ()
      arising from a use of `always'
                   at ...probabilisticModeling.hs:42:13-18
    Possible fix:
      add (Ord a) to the context of the type signature for `return'
    In the expression: always
    In the definition of `return': return = always
    In the instance declaration for `Monad Distribution'

事实上 always return 是: (Ord a) => a -> Distribution a . 有什么办法可以给我一个单子吗 Distribution ,但强制执行约束 (Ord a) 在这个单子上?我试过:

instance Monad Distribution where
    (>>=) = bind
    return :: (Ord a) => a -> Distribution a = always

但我得到了一个错误:

...probabilisticModeling2.hs:48:4:
    Pattern bindings (except simple variables) not allowed in instance declarations
      return :: (Ord a) => a -> Distribution a = always
Failed, modules loaded: none.

所以有一种方法可以得到单子 我是 A. 具有约束,例如 Ord a ?

谢谢