Haskell - `scanl` 在这个表达式中做了什么

Question

New to Haskell and Functional Programming

In Haskell (ghci) what does scanl do in the following expressions?

• scanl (+) 0 [1,3..]
• scanl (*) 1 [1..]

At first I thought it makes an infinite list of odd numbers where it successively adds them however this doesn't sound right. What does both those expressions do?

Thank you

Answer 1

scanl is just like foldl except for it gives you a list of intermediate results instead of just the final one. Understanding it helps understanding foldl and vice versa. So, for example, whereas foldl (+) 0 finds the sum of all elements of a list, scanl (+) 0 shows you all the intermediate sums leading up to it:

ghci> scanl (+) 0 [1,1,1,2,5]
[0, 1, 2, 3, 5, 10]
-- +1 +1 +1 +2  +5

Similarly, since foldl (++) "" concatenates a bunch of stirngs, scanl (++) "" shows you the intermediate concatenations:

ghci> scanl (++) "" ["foo", "bar", "baz", "quux"]
["", "foo", "foobar", "foobarbaz", "foobarbazquux"]