[project @ 2002-10-09 17:08:18 by malcolm]

[ghc-base.git] / Data / Ix.hs

diff --git a/Data/Ix.hs b/Data/Ix.hs
index 8d4d745..44e113a 100644 (file)
--- a/Data/Ix.hs
+++ b/Data/Ix.hs
@@ -1,27 +1,40 @@
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Data.Ix
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  provisional
 -- Portability :  portable
 --
--- $Id: Ix.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
---
 -- Class of index types.
 --
+-- The "Ix" class is used to map a continuous subrange of values in a type onto
+-- integers. It is used primarily for array indexing (see Section 6
+-- <http://www.haskell.org/onlinelibrary/array.html#arrays>). The "Ix"
+-- class contains the methods range, index, and inRange. The 'index' operation
+-- maps a bounding pair, which defines the lower and upper bounds of the range,
+-- and a subscript, to an integer. The 'range' operation enumerates all
+-- subscripts; the 'inRange' operation tells whether a particular subscript
+-- lies in the range defined by a bounding pair.
+-- 
+-- An implementation is entitled to assume the following laws about these
+-- operations: 
+-- 
+-- >        range (l,u) !! index (l,u) i == i   -- when i is in range
+-- 
+-- >        inRange (l,u) i == i `elem` range (l,u)
+-- 
 -----------------------------------------------------------------------------
-
 module Data.Ix
     (
        Ix
          ( range       -- :: (Ix a) => (a,a) -> [a]
          , index       -- :: (Ix a) => (a,a) -> a   -> Int
          , inRange     -- :: (Ix a) => (a,a) -> a   -> Bool
+         , rangeSize   -- :: (Ix a) => (a,a) -> Int
          )
-    ,  rangeSize       -- :: (Ix a) => (a,a) -> Int
     -- Ix instances:
     --
     --  Ix Char
@@ -34,6 +47,30 @@ module Data.Ix
     --  ...
 
     -- Implementation checked wrt. Haskell 98 lib report, 1/99.
+
+    -- * Deriving Instances of Ix
+    -- | Derived instance declarations for the class 'Ix' are only possible
+    -- for enumerations (i.e. datatypes having only nullary constructors)
+    -- and single-constructor datatypes, including arbitrarily large tuples,
+    -- whose constituent types are instances of 'Ix'. 
+    -- 
+    -- * For an enumeration, the nullary constructors are assumed to be
+    -- numbered left-to-right with the indices being 0 to n-1 inclusive. This
+    -- is the same numbering defined by the 'Enum' class. For example, given
+    -- the datatype: 
+    -- 
+    -- >       data Colour = Red | Orange | Yellow | Green | Blue | Indigo | Violet
+    -- 
+    -- we would have: 
+    -- 
+    -- >       range   (Yellow,Blue)        ==  [Yellow,Green,Blue]
+    -- >       index   (Yellow,Blue) Green  ==  1
+    -- >       inRange (Yellow,Blue) Red    ==  False
+    -- 
+    -- * For single-constructor datatypes, the derived instance declarations
+    -- are as shown for tuples in Figure 1
+    -- <http://www.haskell.org/onlinelibrary/ix.html#prelude-index>.
+
     ) where
 
 import Prelude
@@ -41,3 +78,12 @@ import Prelude
 #ifdef __GLASGOW_HASKELL__
 import GHC.Arr
 #endif
+
+#ifdef __HUGS__
+import Hugs.Prelude( Ix(..) )
+#endif
+
+#ifdef __NHC__
+import Ix (Ix(..))
+#endif
+