Haskell：打印列表元素

Question

I have a list let a = [1,2,3,4]

I want to extract 2 elements at a time to perform computation. Can anyone tell me how this can be achieved? I am new to Haskell.

I am aware of take 2 a . But how can I put this in a loop so that 2 elements are extracted at a time. I am confused.

Answer 1

Here's an example. You are (or should become) familiar with the map function:

map :: (a -> b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs

Here, you might have a function that takes two arguments, and want to apply it to two elements from a list at a time. Here's a function mapTwo that behaves like map , but with a two-argument function. For simplicity, we'll assume the list has an even number of elements.

mapTwo :: (a -> a -> b) -> [a] -> [b]
mapTwo f [] = []
mapTwo f (x1:x2:xs) = f x1 x2 : mapTwo f xs

Answer 2

You can write your own recursive function and pattern-match on elements you are interested in:

f (a:b:list) = doSomeThingWith a b : f list

Make sure to properly match on [] and [a] cases.