Re: [Haskell-cafe] deriving Data.HashTable - stack overflow

I do wish there was a compiler-checked way of specifying a minimum complete definition. On Thu, Aug 8, 2013 at 11:02 AM, Joey Adams wrote:

On Thu, Aug 8, 2013 at 12:22 PM, Lyle Kopnicky wrote:

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So I went to the Data.Hashable page and looked up examples on how to derive a Hashable instance for my datatype:

http://hackage.haskell.org/packages/archive/hashable/latest/doc/html/Data-Ha...

The problem occurs even when using the sample code on the page:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic) import Data.Hashable

data Colour = Red | Green | Blue deriving Generic

instance Hashable Colour

If I then type `hash Red` I get a stack overflow.

I am using the Haskell Platform, so I have hashable-1.1.2.5, but I notice the docs are for hashable-1.2.0.10. If I install 1.2.0.10 though, other code in my project breaks - seems like one part doesn't recognize the instances from another part. So I'll stick with the platform version.

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Generic support was added in hashable-1.2. Before then, the default implementations for `hash` and `hashWithSalt` were written in terms of each other:

hash = hashWithSalt defaultSalt hashWithSalt salt x = salt `combine` hash x

Because you did not give an implementation for either of these, both default implementations were used, leading to a loop.

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