Day 3: Haskell

TLDR : Youtube for the win

So I started watching this youtube series on Haskell, and I must admit it’s pretty good. Looks like I found another source to learn from.

Things I’m learning

• The below code apparently shows us the type of the function. I haven’t been able to replicate it in VSCode, so I’ll check on the terminal later.

:t (+)

• Different ways of defining a recursive factorial function.

fact :: Integer -> Integer
fact 0 = 1
fact n = n * fact(n-1)

fact1 :: Integer -> Integer
fact1 n = if n <= 1 then 1 else n*fact1(n-1)

fact2 :: Integer -> Integer
fact2 n | n <= 1 = 1
        | otherwise = n*fact2(n-1)

I skipped over some usage of a combination of the where clause and guards. I’ll try to implement it later if possible.

• Lists can only have one type (something I figured out through trial and error before).
• Basic List creation function

produceList n m | m<n = []
                | m == n = [m]
                | m > n = n : produceList (n+1) m

Basically, recursively making lists. The semicolon in

n : []

basically means that we’re appending the value n to the beginning of the list.

Something like

1:2:3:4:[]

is equal to

[1,2,3,4]

You’ll also likely get a “Pattern Matches are Inexhaustive” error. Not sure if it’s wrong, but the funtion works anyway for now.

• List functions

Any function that returns a list will return a copy of the list, since data types in haskell are immutable

Head : Returns the first element of the list.

Tail : Returns all the elements of the list other than the first one.

length : Come on.

init : Returns the list without the last element

null : check if list is empty

head (produceList 1 10)
tail (produceList 1 10)
length (produceList 1 10)
init (produceList 1 10) 
null (produceList 1 0))

• List comprehensions!

If you’ve used python, you already know. Nice to see all my knowledge not go to waste.

• Basic List Addition function

sum1 [] = 0
sum1 (x:xs) = x + sum xs
sum2 xs = foldr (*) 1 xs

The first one is basic, however the second one is what is suggested by Haskell (or some extension I installed. Can’t tell, no difference).

Note: Learning Haskell is kind of a pain in the ass, but I must admit that my mind is already racing to stuff I can do cleanly. My college assignments come to mind, and so does my number theory class. I wanted to write a program to calculate that stuff, but I must admit that I felt defeated initially. Looks like I’ll have another go at it.