Myujth

I am new in Haskell and practicing Algorithm in Haskell follow the book "Pearls of Functional Algorithm Design"

This is the algo to find the smallest natural number not in a given finite set X of natural numbers

import Data.List
import Data.Array

minfree xs = if null ([0..b-1] \ us)
 then head ([b..] \ vs)
 else head ([0..b-1] \ us)
 where (us, vs) = (partition (<b) xs)
 b = div (length xs) 2

Pff the compiler error like this

Prelude Data.Array Data.List> :load 01_the_smallest_free_number.hs 
[1 of 1] Compiling Main ( 01_the_smallest_free_number.hs, interpreted )

01_the_smallest_free_number.hs:11:29: error:
 parse error on input ‘=’
 Perhaps you need a 'let' in a 'do' block?
 e.g. 'let x = 5' instead of 'x = 5'
 |
11 | b = div (length xs) 2 
 | ^
Failed, no modules loaded.

Well obveriously add let before b is not the right answer I've tried.

Then I replaced all b to div (length xs) 2 that works, so seems here is the problem to be, but I don't get it

This is bad indentation:

 where (us, vs) = (partition (<b) xs)
 b = div (length xs) 2

Since the second equality is indented more than the first, it continues the first one, as if we wrote

 where (us, vs) = (partition (<b) xs) b = div (length xs) 2

triggering the error.

You instead want:

 where (us, vs) = (partition (<b) xs)
 b = div (length xs) 2

so that both equations are indented in the same way.