Overloaded list notation
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
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
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
fromListN are rebindable).
That said, the
GHC.Exts module exports the
IsList class that can
be used to overload
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
IsList class and its methods are intended to be used in
conjunction with the
OverloadedLists extension. The
function returns the type of items of the structure
l. The fromList
function constructs the structure
l from the given list of
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
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
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
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:
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
to take advantage of the compact representation. Equipped with this capability
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:
https://gitlab.haskell.org/ghc/ghc/issues/5218 http://www.serpentine.com/blog/2012/09/12/the-case-of-the-mysterious-explosion-in-space/ http://email@example.com/msg101412.html
OverloadedLists extension as, implemented above, would not be able to be used on heterogeneous lists, for example, as implemented below:
data HList :: [*] -> * where HNil :: HList ' HCons :: a -> HList xs -> HList (a ': xs)
This is a bit disappointing. However, I'm not really sure how you could make this extension support this use case, even if you added some hacks to the
The current extension can't be used to represent list literals for length-indexed vectors as e.g.
-- (alternatively, GHC.TypeLits.Nat) data Nat = Ze | Su Nat data Vec :: * -> Nat -> * where Nil :: Vec a Ze Cons :: a -> Vec a n -> Vec a (Su n)
as the length-information is not provided in a suitable way.