When you use function name without "&" symbol, Elixir will try to run it. Passing convert/1 without "&" symbol will casue the following error: function convert/0 is undefined or private.

The "&" symbol is named the capture operator. It allows for named functions to be assigned to variables or passed as the arguments. When using it you have to define specific arity of the function.

You can read more about it on https://elixir-lang.org/getting-started/modules-and-functions.html#function-capturing

I think, this solution is not iterating through the alphabet. It gets the binary value for each character in s, and substracts the binary value of a (?a).

woops, replied to wrong comment, how to delete?