Undefined variables¶

At render time, if a variable or path to a variable can not be resolved to a value, an instance of Luoma::UndefinedDrop is used in its place. We can control undefined variable rendering behavior by passing a LuomaUndefinedDrop singleton as the undefined argument when instantiating a new Luoma::Environment.

See https://github.com/jg-rp/luoma-ruby/blob/main/lib/luoma/drops/undefined.rb.

Default undefined¶

The default Undefined type is silent. It renders to the empty string, is falsy when tested for truthiness, and yields an empty iterable when looped over with the {% for %} tag.

Hello {{ nosuchthing }}
{% for thing in nosuchthing %}
    {{ thing }}
{% endfor %}

Hello

Strict undefined¶

Luoma::StrictUndefinedDrop raises a Luoma::UndefinedVariableError in all contexts, even when tested for truthiness.

require "luoma"

env = Luoma::Environment.new(undefined: Luoma::StrictUndefinedDrop)

template = env.parse <<~SOURCE
  {{ nosuchthing }}
SOURCE

puts template.render

# /home/james/projects/luoma-ruby/lib/luoma/drops/undefined.rb:95:in 'Luoma::StrictUndefinedDrop#error': Luoma::UndefinedVariableError: "nosuchthing" is undefined
#   -> "{{ nosuchthing }}":1:4
#   |
# 1 | {{ nosuchthing }}
#   |    ^^^^^^^^^^^ "nosuchthing" is undefined

Falsy strict undefined¶

Luoma::FalsyStrictUndefinedDrop is similar to Luoma::StrictUndefinedDrop, but can be tested for truthiness and equality without raising an exception.

require "luoma"

env = Luoma::Environment.new(undefined: Luoma::FalsyStrictUndefinedDrop)

template = env.parse <<~SOURCE
  {% if nosuchthing %}TRUE{% else %}FALSE{% endif %}
SOURCE

puts template.render

FALSE