Can't make a new data type Hashable

haskell

Solution

The hackage docs for Data.Hashable states that the minimal implementation of Hashable is the function `hashWithSalt` — check out the documentation under the typeclass declaration (`class Hashable a where`).

So if you change your function to `hashWithSalt`, everything should work:

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

Problem

Attempt to compile the following with ghc 7.8.2 on OS X 10.9.3 ``` 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) ``` fails with ``` 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)’ ``` What is wrong?

Original source