Ruby erb template 'or' operator - erb

Question

I have a puppet manifest which uses an erb template.
This is the correct syntax according to the documentation, and it works perfectly.
<% if foo != bar %>
derp
<% else %>
herp
<% end %>
How can I combine the if statement with an 'or' operator?
Here are the following syntaxes that I have tried, however they return an error:
<% if foo != bar or if slurp != burp %>
derp
<% else %>
herp
<% end %>
Error: Could not run: /etc/puppet/modules/gitlab/templates/gitlab.yml.6-7-stable.erb:275: syntax error, unexpected $end, expecting keyword_end
; _erbout.force_encoding(__ENCODING__)
I tried changing <% to <%% since this is mentioned in the puppet docs.
<%% if foo != bar or if slurp != burp %%>
derp
<% else %>
herp
<% end %>
Error: Could not run: /etc/puppet/modules/gitlab/templates/gitlab.yml.6-7-stable.erb:241: syntax error, unexpected keyword_else, expecting $end
; else ; _erbout.concat "\n derp "
^
I've tried splitting the if-s
<% if foo != bar %> or <% if slurp != burp %>
derp
<% else %>
herp
<% end %>
Error: Could not run: /etc/puppet/modules/gitlab/templates/gitlab.yml.6-7-stable.erb:275: syntax error, unexpected $end, expecting keyword_end
; _erbout.force_encoding(__ENCODING__)
^

Answer 1

There is an extra if. The correct syntax is:
<% if foo != bar or slurp != burp %>
derp
<% else %>
herp
<% end %>
But your ones didn't work as your if-else construct was wrong. Read the documentation of if-else construct.

Answer 2

Ruby syntax for if/else blocks is:
if conditional
The conditional can include any logical operators such as 'or', 'and', '&', '|', '&&', '||'.
However, it can not contain another if block as your example has:
if foo != bar or if slurp != burp

Answer 3

ERB is Ruby1. ERB simply extracts the Ruby in the tags and the "inline text" into a mash-up of code and in-line string/output building to yield intermediate Ruby which is then executed.
As such, extracting the actual Ruby (as executed by ERB) looks like:
if foo != bar or if slurp != burp
# some inconsequential ERB magic to output "derp"
else
# some inconsequential ERB magic to output "herp"
end
Now, filling in the non-interesting parts with dummy expressions yields the following. (Ignore that the strings are always truthy-expressions; the problem is a Syntax Error.)
if "hello" or if "world"
else
end
And this yields the expected error message (see ideone demo).
prog.rb:3: syntax error, unexpected $end
Well .. why?
It's because there are two if-expressions2, but only one is closed and the code has been parsed as though grouping parenthesis have been added.
if "hello" or (if "world"
else
end) # <-- no `end` for `if "hello"`!
The fix is then simply to remove the nested if-expression.
# no extra `if` here, so only looking for one `end` below
if "hello" or "world"
else
end
1 A more correct statement would be that the code in ERB tags is Ruby, and ERB internally generates Ruby, using the content of ERB tags directly. This differs from other template engines which introduce their own syntax and parser. ERB is Ruby.
2 Ruby also supports the expr if expr (and if expr then .. end) forms. Regardless, the fundamental issue is the addition of the second unnecessary if.