Scope of variable passed to function

This form rebinds the name number inside the case branch:

def some_fun(number) do
  case 2*number do
    number ->
      # number here is bound to a new value, shadowing the argument named number

This will match ANY integer passed to some_fun.

ON THE OTHER HAND

Using the pin operator says “match this existing value”:

def some_fun(number) do
  case 2*number do
    ^number ->
      # number here is still bound to the argument

That will only match if some_fun is passed 0, since 2*0 == 0

Oh sorry wasn’t responding specifically to your question.

But to answer it: Elixir allows variable re-binding, ie, you can say: number = number+ 1 and not get an error that number is already defined. Like =, -> is also a binding operator, so when you say number -> ok(:perfect), number is rebound. The match on the value of number, you need to use the pin operator.

Ok, I get it, but the question was, why it was not working with case this, matching why do we have to use this ^ pin operator?

People have a weird aversion to cond for some reason that I don’t really understand. Doing pure boolean checks is exactly what it exists for. The case version is essentially the same with unnecessary extra syntax. Using cond gives added information that indeed all clauses are boolean checks. It’s only rare because this doesn’t come up as often in Elixir, but it certainly does come up!

Last question, i cannot find explanatio why this works that way. Do you have explanation or link to resources?

In my experience it’s very rare to use cond but it’s still valid.

Because it’s very rare to use ^number in the case

Yet, You can do as You like

After adding ^ operator it’s right why should I use cond? What is more “idiomatic” whatever this “idiomatic” word mean?

No, your logic is wrong and number might not be what You think…

As soon as there is a match, the rest of the case is discarded

the number in the case is not the number parameter, it is a new number

You probably should write this with cond

aliquot = aliquot_sum(number)
cond do
  aliquot > number -> ...
  aliquot < number -> ...
  aliquot == number -> ...
end