Day 6: Haskell
TLDR : Folding
- foldr/foldl (foldRight and foldLeft)
Folding is a pretty nice operation. Basically, it runs through each element in the list and applies a desired function to it. So what’s the difference between map and foldr? Map treats each element as an individual, and just applies the function to each element only. Folding uses a binary function and recursively applies it to your list using each element as an input.
Better to see it.
foldr :: (a → b → b) → b → [a] → b
If you’re wondering why xs = , print out the result.
foldr gives you 1, while foldl gives you -1.
foldr_output = 1–0 (xs-acc)
foldl_ouput = 0–1 (acc-xs)
The accumulator is the last/first element that you’ll apply the function to.
That’s also when you print out the result, foldr ys = -1
(1–2)–0 = -1
foldl ys = (0–2)-1 = -3
This may seem like semantic wrangling for the sake of it, but it is a good idea to keep in mind.
While foldr doesn’t seem great just yet, think about eigenvectors and eigenvalues. Matrix multiplication seems like a great use for this stuff.
Here are some other uses of foldr
even something like length can be represented by folding.
foldr (\x → (+) 1) 0
foldr (const $ (+) 1) 0
Could probably do a lot more powerful stuff with this, but for now will leave as is.
Note: Map is given as follows
map f = foldr ((:) .f) 
I have no idea what this means. Will read some more and then try to explain.
Just learnt that [X..Y] will print out numbers from X to Y if X≤Y. Otherwise, it returns an empty list.
Note : https://www.youtube.com/watch?v=7sbxVALuuxA
I watched this video and while I understood we could create our own datatypes, I have no clue how to implement them to do whatever. I’ll skip that video and come back later.