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Overloaded list notation
This wiki page documents the design and implementation of the GHC extension for overloading Haskell's list notation.
Let us briefly recap the notation for constructing lists. In Haskell, the list notation can be be used in the following seven ways:
 -- Empty list [x] -- x :  [x,y,z] -- x : y : z :  [x .. ] -- enumFrom x [x,y ..] -- enumFromThen x y [x .. y] -- enumFromTo x y [x,y .. z] -- enumFromThenTo x y z
When the OverloadedLists extension is turned on, the aforementioned seven notations are desugared as follows:
 -- fromListN 0  [x] -- fromListN 1 (x : ) [x,y,z] -- fromListN 3 (x : y : z : ) [x .. ] -- fromList (enumFrom x) [x,y ..] -- fromList (enumFromThen x y) [x .. y] -- fromList (enumFromTo x y) [x,y .. z] -- fromList (enumFromThenTo x y z)
This extension allows programmers to use the list notation for construction of structures like: Set, Map, IntMap, Vector, Text and Array. The following code listing gives a few examples:
['0' .. '9'] :: Set Char [1 .. 10] :: Vector Int [("default",0), (k1,v1)] :: Map String Int ['a' .. 'z'] :: Text
List patterns are also overloaded. 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]) = ...
GHC, during the typechecking and desugaring phases, uses whatever is in scope with the names of fromList, toList and fromListN (i.e., fromList, toList and fromListN are rebindable).
That said, the GHC.Exts module exports the IsList class that can be used to overload fromListN and fromListN for different structures. The type class is defined as follows:
class IsList l where type Item l fromList :: [Item l] -> l toList :: l -> [Item l] fromListN :: Int -> [Item l] -> l fromListN _ = fromList
The IsList class and its methods are intended to be used in conjunction with the OverloadedLists extension. The Item type function returns the type of items of the structure l. The fromList function constructs the structure l from the given list of Item l. The fromListN function takes the input list's length as a hint. Its behaviour should be equivalent to fromList. The hint can be used for more efficient construction of the structure l compared to fromList. If the given hint is not equal to the input list's length the behaviour of fromListN is not specified.
The instances of the IsList class should satisfy the following property:
fromList . toList = id
In the following, we give several example instances of the IsList type class:
instance IsList [a] where type Item [a] = a fromList = id toList = id instance (Ord a) => IsList (Set a) where type Item (Set a) = a fromList = Set.fromList toList = Set.toList instance (Ord k) => IsList (Map k v) where type Item (Map k v) = (k,v) fromList = Map.fromList toList = Map.toList instance IsList (IntMap v) where type Item (IntMap v) = (Int,v) fromList = IntMap.fromList toList = IntMap.toList instance IsList Text where type Item Text = Char fromList = Text.pack toList = Text.unpack instance IsList (Vector a) where type Item (Vector a) = a fromList = Vector.fromList toList = Vector.toList fromListN = Vector.fromListN
Further GHC improvements/extensions that may benefit OverloadedLists
Currently, the IsList class is not accompanied with defaulting rules. Although feasible, not much thought has gone into how to specify the meaning of the default declarations like: default ([a])
The current implementation of the OverloadedLists extension can be improved by handling the lists that are only populated with literals in a special way. More specifically, the compiler could allocate such lists statically using a compact representation and allow IsList instances to take advantage of the compact representation. Equipped with this capability the OverloadedLists extension will be in a good position to subsume the OverloadedStrings extension (currently, as a special case, string literals benefit from statically allocated compact representation).
Somewhat related discussions:
http://hackage.haskell.org/trac/ghc/ticket/5218 http://www.serpentine.com/blog/2012/09/12/the-case-of-the-mysterious-explosion-in-space/ http://firstname.lastname@example.org/msg101412.html