Changes between Version 1 and Version 2 of OverloadedLists

Timestamp:

11/06/12 06:40:32 (7 months ago)

Author:

giorgidze

Comment:

--

OverloadedLists

@@ -111 +111 @@
 }}}
 
-== TODO ==
-
-== Overloading Pattern Matching on Lists ==
-...
+== TODO: Overloading Pattern Matching on Lists ==
+
+One way to overload list patterns is to replace the {{{FromList}}} class from
+the previous section with the {{{OverloadedLists}}} class defined as follows:
+
+{{{
+class OverloadedLists l where
+  type Item l
+  fromList  :: [Item l] -> l
+  toList    :: l -> [Item l]
+
+  fromListN :: Int -> [Item l] -> l
+  fromListN _ = fromList
+}}}
+
+This class allows us to view the structure {{{l}}} as a list. Now, the
+{{{toList}}} can be used to overload the list notation in patters.
+For example, When the {{{OverloadedLists}}} extension is turned on, the
+definitions
+
+{{{
+f [] = ...
+g [x,y,z] = ...
+}}}
+
+will be treated as
+
+{{{
+f (toList -> []) = ...
+g (toList -> [x,y,z]) = ...
+}}}
+
+Here, just like for expression, we propose to overload list patterns that use
+square brackets. The {{{:}}} infix operator will remain list specific both in
+expressions and patterns. In other words, {{{:}}} is not overloaded.
+
+The instances of the {{{OverloadedLists}}} class should satisfy the following
+property:
+
+{{{
+fromList . toList = id
+}}}
+
+The example {{{FromList}}} instances from the previous section can be extended
+into the {{{OverloadedLists}}} instances as follows:
+
+{{{
+instance OverloadedLists [a] where
+  type Item [a] = a
+  fromList = id
+  toList = id
+
+instance (Ord a) => OverloadedLists (Set a) where
+  type Item (Set a) = a
+  fromList = Set.fromList
+  toList = Set.toList
+
+instance (Ord k) => OverloadedLists (Map k v) where
+  type Item (Map k v) = (k,v)
+  fromList = Map.fromList
+  toList = Map.toList
+
+instance OverloadedLists (IntMap v) where
+  type Item (IntMap v) = (Int,v)
+  fromList = IntMap.fromList
+  toList = IntMap.toList
+
+instance OverloadedLists Text where
+  type Item Text = Char
+  fromList = Text.pack
+  toList = Text.unpack
+
+instance OverloadedLists (Vector a) where
+  type Item (Vector a) = a
+  fromList  = Vector.fromList
+  fromListN = Vector.fromListN
+  toList = Vector.toList
+}}}
+
+== Further Improvements ==
 
 == Defaulting ==