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<pre><a name="line-1"></a><span class='hs-comment'>------------------------------------------------------------------------</span>
<a name="line-2"></a><span class='hs-comment'>-- |</span>
<a name="line-3"></a><span class='hs-comment'>-- Module : Data.Hashable.Class</span>
<a name="line-4"></a><span class='hs-comment'>-- Copyright : (c) Milan Straka 2010</span>
<a name="line-5"></a><span class='hs-comment'>-- (c) Johan Tibell 2011</span>
<a name="line-6"></a><span class='hs-comment'>-- (c) Bryan O'Sullivan 2011, 2012</span>
<a name="line-7"></a><span class='hs-comment'>-- License : BSD-style</span>
<a name="line-8"></a><span class='hs-comment'>-- Maintainer : johan.tibell@gmail.com</span>
<a name="line-9"></a><span class='hs-comment'>-- Stability : provisional</span>
<a name="line-10"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-11"></a><span class='hs-comment'>--</span>
<a name="line-12"></a><span class='hs-comment'>-- Lifting of the 'Hashable' class to unary and binary type constructors.</span>
<a name="line-13"></a><span class='hs-comment'>-- These classes are needed to express the constraints on arguments of</span>
<a name="line-14"></a><span class='hs-comment'>-- types that are parameterized by type constructors. Fixed-point data</span>
<a name="line-15"></a><span class='hs-comment'>-- types and monad transformers are such types.</span>
<a name="line-16"></a>
<a name="line-17"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Hashable</span><span class='hs-varop'>.</span><span class='hs-conid'>Lifted</span>
<a name="line-18"></a> <span class='hs-layout'>(</span> <span class='hs-comment'>-- * Type Classes</span>
<a name="line-19"></a> <span class='hs-conid'>Hashable1</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span>
<a name="line-20"></a> <span class='hs-layout'>,</span> <span class='hs-conid'>Hashable2</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span>
<a name="line-21"></a> <span class='hs-comment'>-- * Auxiliary Functions</span>
<a name="line-22"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>hashWithSalt1</span>
<a name="line-23"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>hashWithSalt2</span>
<a name="line-24"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>defaultLiftHashWithSalt</span>
<a name="line-25"></a> <span class='hs-comment'>-- * Motivation</span>
<a name="line-26"></a> <span class='hs-comment'>-- $motivation</span>
<a name="line-27"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-28"></a>
<a name="line-29"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Hashable</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-30"></a>
<a name="line-31"></a><span class='hs-comment'>-- $motivation</span>
<a name="line-32"></a><span class='hs-comment'>--</span>
<a name="line-33"></a><span class='hs-comment'>-- This type classes provided in this module are used to express constraints</span>
<a name="line-34"></a><span class='hs-comment'>-- on type constructors in a Haskell98-compatible fashion. As an example, consider</span>
<a name="line-35"></a><span class='hs-comment'>-- the following two types (Note that these instances are not actually provided</span>
<a name="line-36"></a><span class='hs-comment'>-- because @hashable@ does not have @transformers@ or @free@ as a dependency):</span>
<a name="line-37"></a><span class='hs-comment'>--</span>
<a name="line-38"></a><span class='hs-comment'>-- > newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }</span>
<a name="line-39"></a><span class='hs-comment'>-- > data Free f a = Pure a | Free (f (Free f a))</span>
<a name="line-40"></a><span class='hs-comment'>--</span>
<a name="line-41"></a><span class='hs-comment'>-- The 'Hashable1' instances for @WriterT@ and @Free@ could be written as:</span>
<a name="line-42"></a><span class='hs-comment'>--</span>
<a name="line-43"></a><span class='hs-comment'>-- > instance (Hashable w, Hashable1 m) => Hashable1 (WriterT w m) where</span>
<a name="line-44"></a><span class='hs-comment'>-- > liftHashWithSalt h s (WriterT m) =</span>
<a name="line-45"></a><span class='hs-comment'>-- > liftHashWithSalt (liftHashWithSalt2 h hashWithSalt) s m</span>
<a name="line-46"></a><span class='hs-comment'>-- > instance Hashable1 f => Hashable1 (Free f) where</span>
<a name="line-47"></a><span class='hs-comment'>-- > liftHashWithSalt h = go where</span>
<a name="line-48"></a><span class='hs-comment'>-- > go s x = case x of</span>
<a name="line-49"></a><span class='hs-comment'>-- > Pure a -> h s a</span>
<a name="line-50"></a><span class='hs-comment'>-- > Free p -> liftHashWithSalt go s p</span>
<a name="line-51"></a><span class='hs-comment'>--</span>
<a name="line-52"></a><span class='hs-comment'>-- The 'Hashable' instances for these types can be trivially recovered with</span>
<a name="line-53"></a><span class='hs-comment'>-- 'hashWithSalt1':</span>
<a name="line-54"></a><span class='hs-comment'>--</span>
<a name="line-55"></a><span class='hs-comment'>-- > instance (Hashable w, Hashable1 m, Hashable a) => Hashable (WriterT w m a) where</span>
<a name="line-56"></a><span class='hs-comment'>-- > hashWithSalt = hashWithSalt1</span>
<a name="line-57"></a><span class='hs-comment'>-- > instance (Hashable1 f, Hashable a) => Hashable (Free f a) where</span>
<a name="line-58"></a><span class='hs-comment'>-- > hashWithSalt = hashWithSalt1</span>
<a name="line-59"></a>
<a name="line-60"></a><span class='hs-comment'>--</span>
<a name="line-61"></a><span class='hs-comment'>-- $discussion</span>
<a name="line-62"></a><span class='hs-comment'>--</span>
<a name="line-63"></a><span class='hs-comment'>-- Regardless of whether 'hashWithSalt1' is used to provide an implementation</span>
<a name="line-64"></a><span class='hs-comment'>-- of 'hashWithSalt', they should produce the same hash when called with</span>
<a name="line-65"></a><span class='hs-comment'>-- the same arguments. This is the only law that 'Hashable1' and 'Hashable2'</span>
<a name="line-66"></a><span class='hs-comment'>-- are expected to follow.</span>
<a name="line-67"></a><span class='hs-comment'>--</span>
<a name="line-68"></a><span class='hs-comment'>-- The typeclasses in this module only provide lifting for 'hashWithSalt', not</span>
<a name="line-69"></a><span class='hs-comment'>-- for 'hash'. This is because such liftings cannot be defined in a way that</span>
<a name="line-70"></a><span class='hs-comment'>-- would satisfy the @liftHash@ variant of the above law. As an illustration</span>
<a name="line-71"></a><span class='hs-comment'>-- of the problem we run into, let us assume that 'Hashable1' were</span>
<a name="line-72"></a><span class='hs-comment'>-- given a 'liftHash' method:</span>
<a name="line-73"></a><span class='hs-comment'>--</span>
<a name="line-74"></a><span class='hs-comment'>-- > class Hashable1 t where</span>
<a name="line-75"></a><span class='hs-comment'>-- > liftHash :: (Int -> a) -> t a -> Int</span>
<a name="line-76"></a><span class='hs-comment'>-- > liftHashWithSalt :: (Int -> a -> Int) -> Int -> t a -> Int</span>
<a name="line-77"></a><span class='hs-comment'>--</span>
<a name="line-78"></a><span class='hs-comment'>-- Even for a type as simple as 'Maybe', the problem manifests itself. The</span>
<a name="line-79"></a><span class='hs-comment'>-- 'Hashable' instance for 'Maybe' is:</span>
<a name="line-80"></a><span class='hs-comment'>--</span>
<a name="line-81"></a><span class='hs-comment'>-- > distinguisher :: Int</span>
<a name="line-82"></a><span class='hs-comment'>-- > distinguisher = ...</span>
<a name="line-83"></a><span class='hs-comment'>-- ></span>
<a name="line-84"></a><span class='hs-comment'>-- > instance Hashable a => Hashable (Maybe a) where</span>
<a name="line-85"></a><span class='hs-comment'>-- > hash Nothing = 0</span>
<a name="line-86"></a><span class='hs-comment'>-- > hash (Just a) = distinguisher `hashWithSalt` a</span>
<a name="line-87"></a><span class='hs-comment'>-- > hashWithSalt s Nothing = ...</span>
<a name="line-88"></a><span class='hs-comment'>-- > hashWithSalt s (Just a) = ...</span>
<a name="line-89"></a><span class='hs-comment'>--</span>
<a name="line-90"></a><span class='hs-comment'>-- The implementation of 'hash' calls 'hashWithSalt' on @a@. The hypothetical</span>
<a name="line-91"></a><span class='hs-comment'>-- @liftHash@ defined earlier only accepts an argument that corresponds to</span>
<a name="line-92"></a><span class='hs-comment'>-- the implementation of 'hash' for @a@. Consequently, this formulation of</span>
<a name="line-93"></a><span class='hs-comment'>-- @liftHash@ would not provide a way to match the current behavior of 'hash'</span>
<a name="line-94"></a><span class='hs-comment'>-- for 'Maybe'. This problem gets worse when 'Either' and @[]@ are considered.</span>
<a name="line-95"></a><span class='hs-comment'>-- The solution adopted in this library is to omit @liftHash@ entirely.</span>
<a name="line-96"></a>
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