Check if a variable is undef in puppet template - erb

Question

What is the proper way to check if a variable is undef in a puppet template?
In the manifest the variable is defined as follows
$myvar = undef
How is this checked in the template?
Is saw the following two variants
<% if #myvar -%>
<% end -%>
and
<% if not #myvar.nil? and #myvar -%>
<% end -%>
They both seem to work in my case, but I wonder if the first approach fails in on certain cases?

Answer 1

The Puppet documentation (at the time of writing this answer) explains it very well: https://puppet.com/docs/puppet/latest/lang_template_erb.html#concept-5365
Since undef is not the same as false, just using an if is not a good way to check for it. Also when a variable is defined, but has a value of false or nil it is also impossible to check with a simple if.
This is why you want to use scope.lookupvar(‘variable’) and check its return value for :undef or :undefined (or nil) to know if it was set to undef, or never set at all.

Answer 2

I'd say the check depends on whether you want an alternative when the variable is not defined.
I'm using the following rules:
Required variable
Check in your puppet script whether the variable contains the expected value before even considering template rendering:
if $myvar == undef {
fail {"You really must set myvar, seriously."}
}
if ! $anothervar {
fail {"anothervar is false, undefined or empty."}
}
You can make your life easier by setting the type of parameters explicitly. This spares you type comparisons and conversions.
In your template you simply write the variables then:
<%= #myvar %>
<%= #anothervar %>
Optional variable that must be defined
If you assume the variable is defined, you can treat it as boolean.
The mapping is as follows (source):
falsey: empty string, false, undef
truthy: everything else
In Puppet >=4:
falsey: false, undef
truthy: everything else
Examples:
print 'something' if #myvar evaluates to true, otherwise 'something else'.
<% if #myvar %>something<% else %>something else<% end %>
print 'something' if #myvar evaluates to true
<% if #myvar %>something<% end %>
print #myvar if it evaluates to true, otherwise 'alternative' %>
<%= #myvar ? #myvar : 'alternative' %>
Optional variable that may be defined
If you are not sure a variable is even defined and don't want to make wrong assumptions, check it in the template.
Examples:
print 'something' followed by #myvar if #myvar is defined and not empty
<% if defined?(#myvar) && ! #myvar.empty? %>something<%= #myvar %><% end %>
print #myvar if it's defined and greater than 10
<%= #myvar if defined?(#myvar) && #myvar > 10 %>

Answer 3

The first one should work like a charm, it's what is being taught in the courses as well.
Number two seems... redundant.