Question

尝试在OS X 10.9.3

上使用ghc 7.8.2编译以下内容

import Data.Hashable (Hashable, hash)

data Edge v = Edge v v deriving (Show)

instance (Eq v) => Eq (Edge v) where
  Edge x1 x2 == Edge y1 y2 =
    x1 == y1 && x2 == y2 || x1 == y2 && x2 == y1

instance (Hashable v) => Hashable (Edge v) where
  hash (Edge x1 x2) = (hash x1) + (hash x2)

失败

Could not deduce (hashable-1.2.1.0:Data.Hashable.Class.GHashable
                    (GHC.Generics.Rep (Edge v)))
  arising from a use of ‘hashable-1.2.1.0:Data.Hashable.Class.$gdmhashWithSalt’
from the context (Hashable v)
  bound by the instance declaration at src/MinCut.hs:12:10-42
In the expression:
  hashable-1.2.1.0:Data.Hashable.Class.$gdmhashWithSalt
In an equation for ‘hashWithSalt’:
    hashWithSalt
      = hashable-1.2.1.0:Data.Hashable.Class.$gdmhashWithSalt
In the instance declaration for ‘Hashable (Edge v)’

有什么问题？

Answer 1

Data.Hashable的hackage文档声明Hashable的最小实现是函数hashWithSalt - 查看类型类声明（class Hashable a where）下的文档。

因此，如果您将功能更改为hashWithSalt，则一切都应该有效：

instance (Hashable v) => Hashable (Edge v) where
  hashWithSalt s (Edge x1 x2) = s + (hash x1) + (hash x2)

无法使新数据类型Hashable

1 个答案: