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metaprogrammingcrystal-lang

How to check if a constant is defined in Crystal

I need to verify if a constant is defined to do a conditional.

I was trying this but "defined" method not exists on this language:

if defined(constant)
  value = :foo
else
  value = :bar
end
Solution

  • You can use macro and TypeNode#has_constant?:

    FOO = 1

value = nil
{% if @type.has_constant? "FOO" %}
  value = :foo
{% else %}
  value = :bar
{% end %}

pp value #=> :foo

    Or even better, you can write a short custom macro for this:

    macro toplevel_constant_defined?(c)
  {{ @type.has_constant? c }}
end

pp toplevel_constant_defined? "FOO" # => true
pp toplevel_constant_defined? "BAR" # => false

    Note: as mentioned by Jonne Haß, you only ever should need this in advanced macro programming, everywhere else it's a huge code smell, regardless of the language used.