Vfrdtyky

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.