From 466922c82316cecf75a3cc7b31e9331a60c40625 Mon Sep 17 00:00:00 2001
From: simonmar <unknown>
Date: Mon, 3 Jun 2002 14:16:56 +0000
Subject: [haddock @ 2002-06-03 14:16:56 by simonmar] Example from the paper

---
 examples/Hash.hs | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 examples/Hash.hs

(limited to 'examples/Hash.hs')

diff --git a/examples/Hash.hs b/examples/Hash.hs
new file mode 100644
index 00000000..b399b129
--- /dev/null
+++ b/examples/Hash.hs
@@ -0,0 +1,45 @@
+{- |
+  Implementation of fixed-size hash tables, with a type 
+  class for constructing hash values for structured types.
+-}
+module Hash (
+  -- * The @HashTable@ type
+  HashTable,
+
+  -- ** Operations on @HashTable@s
+  new, insert, lookup,
+
+  -- * The @Hash@ class
+  Hash(..),
+ ) where
+
+import Array
+
+-- | A hash table with keys of type @key@ and values of type @val@.
+-- The type @key@ should be an instance of 'Eq'.
+data HashTable key val = HashTable Int (Array Int [(key,val)])
+
+-- | Builds a new hash table with a given size
+new :: (Eq key, Hash key) => Int -> IO (HashTable key val)
+
+-- | Inserts a new element into the hash table
+insert :: (Eq key, Hash key) => key -> val -> IO ()
+
+-- | Looks up a key in the hash table, returns @'Just' val@ if the key
+-- was found, or 'Nothing' otherwise.
+lookup 	:: Hash key => key -> IO (Maybe val)
+
+-- | A class of types which can be hashed.
+class Hash a where
+   -- | hashes the value of type @a@ into an 'Int'
+   hash :: a -> Int
+
+instance Hash Int where
+   hash = id
+
+instance Hash Float where
+   hash = trunc
+
+instance (Hash a, Hash b) => Hash (a,b) where
+   hash (a,b) = hash a `xor` hash b
+
-- 
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