Haskell Language Common GHC Language Extensions OverloadedLists
Example
added in GHC 7.8.
OverloadedLists, similar to OverloadedStrings, allows list literals to be desugared as follows:
[] -- fromListN 0 []
[x] -- fromListN 1 (x : [])
[x .. ] -- fromList (enumFrom x)
This comes handy when dealing with types such as Set, Vector and Maps.
['0' .. '9'] :: Set Char
[1 .. 10] :: Vector Int
[("default",0), (k1,v1)] :: Map String Int
['a' .. 'z'] :: Text
IsList class in GHC.Exts is intended to be used with this extension.
IsList is equipped with one type function, Item, and three functions, fromList :: [Item l] -> l, toList :: l -> [Item l] and fromListN :: Int -> [Item l] -> l where fromListN is optional. Typical implementations are:
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
Examples taken from OverloadedLists – GHC.
