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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Ah, thanks, folks! I'll just implement my own hashing by generating a string and calling the hash function on that. That's what I was doing in the old version of my code, anyway. It's just that in the core Data.HashTable, you had to provide a hash function, so the point where I used the hash table was able to call a string conversion function that was defined elsewhere. With the hashtable package, you have to define a Hashable instance in the same package as your datatype definition - which is not where the string conversion is implemented. Calling the string conversion function leads to a cyclic dependency that I wanted to avoid. So I'll have to maybe move it to live with the datatype definition, or duplicate it, or use some other means of hashing. In other words, package A defines datatype A. Package B defines A -> String. Package C creates a HashTable that indexes by As, and also imports package B, so it can build a hash function as the composition of the string conversion and the provided string hashing. But now I need to define the Hashable instance in A, which doesn't have access to package B, since package B also depends on A, and I don't like circular dependencies. - Lyle

Here's another way to do it: data ValType = FloatType | IntType | StringType deriving (Show,Eq) instance Hashable ValType where hash FloatType = 0 hash IntType = 1 hash StringType = 2 data VarName = VarName ValType String deriving (Show,Eq) instance Hashable VarName where hash (VarName t n) = hash (t, n)