Haskell Language Lists List basics
Example
The type constructor for lists in the Haskell Prelude is []. The type declaration for a list holding values of type Int is written as follows:
xs :: [Int] -- or equivalently, but less conveniently,
xs :: [] Int
Lists in Haskell are homogeneous sequences, which is to say that all elements must be of the same type. Unlike tuples, list type is not affected by length:
[1,2,3] :: [Int]
[1,2,3,4] :: [Int]
Lists are constructed using two constructors:
-
[]constructs an empty list. -
(:), pronounced "cons", prepends elements to a list. Consingx(a value of typea) ontoxs(a list of values of the same typea) creates a new list, whose head (the first element) isx, and tail (the rest of the elements) isxs.
We can define simple lists as follows:
ys :: [a]
ys = []
xs :: [Int]
xs = 12 : (99 : (37 : []))
-- or = 12 : 99 : 37 : [] -- ((:) is right-associative)
-- or = [12, 99, 37] -- (syntactic sugar for lists)
Note that (++), which can be used to build lists is defined recursively in terms of (:) and [].
