This module defines a class, Hashable, for types that can be converted to a hash value. This class exists for the benefit of hashing-based data structures. The module provides instances for basic types and a way to combine hash values.

The hash function should be as collision-free as possible, which means that the hash function must map the inputs to the hash values as evenly as possible.

The functions below can be used when creating new instances of Hashable. For example, for many string-like types the hashWithSalt method can be defined in terms of either hashPtrWithSalt or hashByteArrayWithSalt. Here's how you could implement an instance for the ByteString data type, from the bytestring package:

 import qualified Data.ByteString as B
 import qualified Data.ByteString.Internal as B
 import qualified Data.ByteString.Unsafe as B
 import Data.Hashable
 import Foreign.Ptr (castPtr)

 instance Hashable B.ByteString where
     hashWithSalt salt bs = B.inlinePerformIO $
                            B.unsafeUseAsCStringLen bs $ \(p, len) ->
                            hashPtrWithSalt p (fromIntegral len) salt

Use hashWithSalt to create a hash value from several values, using this recipe:

 instance (Hashable a1, Hashable a2) => Hashable (a1, a2) where
     hash (a1, a2) = hash a1 `hashWithSalt` a2

You can combine multiple hash values using combine, using this recipe:

 combineTwo h1 h2 = 17 `combine` h1 `combine` h2

As zero is a left identity of combine, a nonzero seed is used so that the number of combined hash values affects the final result, even if the first hash values are zero. The value 17 is arbitrary.

When possible, use hashWithSalt to compute a hash value from multiple values instead of computing separate hashes for each value and then combining them using combine.