foldr and foldl

Question 1

Explain the map function

Answer 1

loop over list element-by-element, append new element to new list

Question 2

Explain the foldl function

Answer 2

loop over list element-by-element, update accumulator using current accumulator and element

Question 3

Explain the foldr function

Answer 3

foldr : loop over reverse list element-by-element, update accumulator using current accumulator and element

Question 4

Give the map function definition

Answer 4

map :: (a -> b) -> [a] -> [b]

Question 5

Give the foldl function definition

Answer 5

foldl :: (b -> a -> b) -> b -> [a] -> b

Question 6

Give the foldr function definition

Answer 6

foldr :: (a -> b -> b) -> b -> [a] -> b

Question 7

What is the difference between foldl and foldr

Answer 7

foldl evaluates from left to right (left-associative)

foldr evaluates from right to left (right-associative)