Where is the indentation/layout error in my Haskell code?

Question

I am trying to finish up a simple homework assignment in Haskell for a class at my university, but I cannot figure out why my code won't compile:

-- Comments
module Main where

main :: IO ()
main = do
  n <- readLn
  print (fac n)
  print (facList n)
  print (sumFacs n)
  print (fibonacci n)

-- Aufgabe 2 (a):
fac :: Int -> Int
let
  fac 0 = 1
  fac i = i * fac(i - 1)

-- Aufgabe 2 (b):
facList :: Int -> Int -> [Int]
let
  facList x y = [fac m | m <- [x..y]]

sumFacs :: Int -> Int -> Int
let
  sumFacs x y = sum (facList x y)

-- Aufgabe 3:
fibonacci :: Int -> Int
let
  fibonacci 0 = 1
  fibonacci 1 = 1
  fibonacci i = fibonacci (i - 1) + fibonacci (i - 2)

When I attempt to compile the above code using the Glasgow compiler, I get the following error message:

Uebung01.hs:19:1: error:
    parse error (possibly incorrect indentation or mismatched brackets)
   |
19 | facList :: Int -> Int -> [Int]
   | ^

All of the functions work in interactive mode. Sorry for posting such a simple question, but I am completely new to Haskell and am really struggling to understand how the whitespace rules work. I have looked at answers to similar questions, but I'm still unable to find my mistake. Thanks for reading.

Answer 1

A let block [Haskell-report] expects an in to specify expression. In your fac function you define a let block, but without an in, this is used to define locally scoped variable(s) that you can then use in the in clause. You however do not need a let here, you can define fac as:

fac :: Int -> Int
fac 0 = 1
fac i = i * fac (i - 1)

You need to refactor other functions in a similar manner.