Organizing your code in files

Writing code in a single file is fine for some quick tests or very small applications, but anything else will eventually need to be organized in multiple files. There is always the main file, which is the one you pass to the crystal run or the crystal build command, but this file can reference code in other files with the require keyword. Compilation will always begin by analyzing this main file and then analyzing any file it references, and so on, recursively.

Let's analyze an example:

1. First, create a file named factorial.cr:

   def factorial(n)
     (1..n).product
   end

2. Then, create a file named program.cr:

   require "./factorial"
   (1..10).each do |i|
     puts "#{i}! = #{factorial(i)}"
   end

In this example, require "./factorial" will search for a file named factorial.cr in the same folder as program.cr and import everything it defines. There is no way to select only part of what the required...