Fresh variables

Once macros generate code, they are parsed with a regular Crystal parser where local variables in the context of the macro invocations are assumed to be defined.

This is better understood with an example:

  1. macro update_x
  2.   x = 1
  3. end
  5. x = 0
  6. update_x
  7. x # => 1

This can sometimes be useful to avoid repetitive code by deliberately reading/writing local variables, but can also overwrite local variables by mistake. To avoid this, fresh variables can be declared with %name:

  1. macro dont_update_x
  2.   %x = 1
  3.   puts %x
  4. end
  6. x = 0
  7. dont_update_x # outputs 1
  8. x             # => 0

Using %x in the above example, we declare a variable whose name is guaranteed not to conflict with local variables in the current scope.

Additionally, fresh variables with respect to some other AST node can be declared with %var{key1, key2, ..., keyN}. For example:

  1. macro fresh_vars_sample(*names)
  2.   # First declare vars
  3.   {% for name, index in names %}
  4.     print "Declaring: ", stringify(%name{index}), '\n'
  5.     %name{index} = {{index}}
  6.   {% end %}
  8.   # Then print them
  9.   {% for name, index in names %}
  10.     print stringify(%name{index}), ": ", %name{index}, '\n'
  11.   {% end %}
  12. end
  14. macro stringify(var)
  15.   {{ var.stringify }}
  16. end
  18. fresh_vars_sample a, b, c
  20. # Sample output:
  21. # Declaring: __temp_255
  22. # Declaring: __temp_256
  23. # Declaring: __temp_257
  24. # __temp_255: 0
  25. # __temp_256: 1
  26. # __temp_257: 2

In the above example, three indexed variables are declared, assigned values, and then printed, displaying their corresponding indices.