Haskell Language Lists
Syntax
-
empty list constructor
[] :: [a] -
non-empty list constructor
(:) :: a -> [a] -> [a] -
head - returns the first value of a list
head :: [a] -> a -
last - returns the last value of a list
last :: [a] -> a -
tail - returns a list without the first item
tail :: [a] -> [a] -
init - returns a list without the last item
init :: [a] -> [a] -
xs !! i - return the element at an index i in list xs
(!!) :: Int -> [a] -> a -
take n xs - return new list containing n first elements of the list xs
take :: Int -> [a] -> [a] -
map :: (a -> b) -> [a] -> [b]
-
filter :: (a -> Bool) -> [a] -> [a]
-
(++) :: [a] -> [a]
-
concat :: [[a]] -> [a]
Remarks
- The type
[a]is equivalent to[] a. []constructs the empty list.[]in a function definition LHS, e.g.f [] = ..., is the empty list pattern.x:xsconstructs a list where an elementxis prepended to the listxsf (x:xs) = ...is a pattern match for a non-empty list wherexis the head andxsis the tail.f (a:b:cs) = ...andf (a:(b:cs)) = ...are the same. They are a pattern match for a list of at least two elements where the first element isa, the second element isb, and the rest of the list iscs.f ((a:as):bs) = ...is NOT the same asf (a:(as:bs)) = .... The former is a pattern match for a non-empty list of lists, whereais the head of the head,asis the tail of the head, andbsis the tail.f (x:[]) = ...andf [x] = ...are the same. They are a pattern match for a list of exactly one element.f (a:b:[]) = ...andf [a,b] = ...are the same. They are a pattern match for a list of exactly two elements.f [a:b] = ...is a pattern match for a list of exactly one element where the element is also a list.ais the head of the element andbis the tail of the element.[a,b,c]is the same as(a:b:c:[]). Standard list notation is just syntactic sugar for the(:)and[]constructors.- You can use
all@(x:y:ys)in order to refer to the whole list asall(or any other name you choose) instead of repeating(x:y:ys)again.
