This wiki page documents the design and
implementation of the GHC extension for overloading Haskell's list notation (added in GHC 7.8).
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
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
fromList . toList = id
In the following, we give several example instances of the IsList type
instance IsList [a] where type Item [a] = a fromList = id toList = idinstance (Ord a) => IsList (Set a) where type Item (Set a) = a fromList = Set.fromList toList = Set.toListinstance (Ord k) => IsList (Map k v) where type Item (Map k v) = (k,v) fromList = Map.fromList toList = Map.toListinstance IsList (IntMap v) where type Item (IntMap v) = (Int,v) fromList = IntMap.fromList toList = IntMap.toListinstance IsList Text where type Item Text = Char fromList = Text.pack toList = Text.unpackinstance 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).