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 most standard types. Efficient instances for other types can be generated automatically and effortlessly using the generics support in GHC 7.2 and above.

The easiest way to get started is to use the hash function. Here is an example session with ghci.

ghci> import Data.Hashable
ghci> hash "foo"
60853164

Applications that use hash-based data structures to store input from untrusted users can be susceptible to "hash DoS", a class of denial-of-service attack that uses deliberately chosen colliding inputs to force an application into unexpectedly behaving with quadratic time complexity.

At this time, the string hashing functions used in this library are susceptible to such attacks and users are recommended to either use a Map to store keys derived from untrusted input or to use a hash function (e.g. SipHash) that's resistant to such attacks. A future version of this library might ship with such hash functions.

There are two ways to create new instances: by deriving instances automatically using GHC's generic programming support or by writing instances manually.

Beginning with GHC 7.2, the recommended way to make instances of Hashable for most types is to use the compiler's support for automatically generating default instances.

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic)
import Data.Hashable

data Foo a = Foo a String
             deriving (Eq, Generic)

instance Hashable a => Hashable (Foo a)

data Colour = Red | Green | Blue
              deriving Generic

instance Hashable Colour

If you omit a body for the instance declaration, GHC will generate a default instance that correctly and efficiently hashes every constructor and parameter.

Suppose you intend to use the generic machinery to automatically generate a Hashable instance.

data Oops = Oops
     -- forgot to add "deriving Generic" here!

instance Hashable Oops

And imagine that, as in the example above, you forget to add a "deriving Generic" clause to your data type. At compile time, you will get an error message from GHC that begins roughly as follows:

No instance for (GHashable (Rep Oops))

This error can be confusing, as GHashable is not exported (it is an internal typeclass used by this library's generics machinery). The correct fix is simply to add the missing "deriving Generic".

To maintain high quality hashes, new Hashable instances should be built using existing Hashable instances, combinators, and hash functions.

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

Hash constructors with multiple fields by chaining hashWithSalt:

data Date = Date Int Int Int

instance Hashable Date where
    hashWithSalt s (Date yr mo dy) =
        s `hashWithSalt`
        yr `hashWithSalt`
        mo `hashWithSalt` dy

If you need to chain hashes together, use hashWithSalt and follow this recipe:

combineTwo h1 h2 = h1 `hashWithSalt` h2

For a type with several value constructors, there are a few possible approaches to writing a Hashable instance.

If the type is an instance of Enum, the easiest path is to convert it to an Int, and use the existing Hashable instance for Int.

data Color = Red | Green | Blue
             deriving Enum

instance Hashable Color where
    hashWithSalt = hashUsing fromEnum

If the type's constructors accept parameters, it is important to distinguish the constructors. To distinguish the constructors, add a different integer to the hash computation of each constructor:

data Time = Days Int
          | Weeks Int
          | Months Int

instance Hashable Time where
    hashWithSalt s (Days n)   = s `hashWithSalt`
                                (0::Int) `hashWithSalt` n
    hashWithSalt s (Weeks n)  = s `hashWithSalt`
                                (1::Int) `hashWithSalt` n
    hashWithSalt s (Months n) = s `hashWithSalt`
                                (2::Int) `hashWithSalt` n