I was looking over some code that I thought should have generated an error:

def something
  # ... blah blah blah

  if some_condition_never_run
    foobar = some_method_making_a_value
  else

  call_other_method foobar  
end

In this case I thought foobar would have been undefined at the point where call_other_method() is run, but no! The value nil was being passed in. I somehow expected instead the error: undefined local variable or method `foobar’. Why didn’t this make an exception? Scoping.

Check this:

Even though the stuff in the if block isn’t executed the fact is that foobar is assigned at least once in that localized scope. Therefore when Ruby runs call_other_method() it knows it’s a variable (not a method) and can safely return nil. In the above example, function c‘s proc creates that local variable context within c() and so c‘s foobar doesn’t know about the proc‘s foobar.

It makes sense when you think about it but I still find it awkward.

See also:

• StackOverflow: “I don’t understand ruby local scope”
• StackOverflow: “In Ruby, why after starting irb, foo.nil? says undefined error, and @foo.nil? gives “true”, and @@wah.nil? gives error again?”

I had something I was doing repetitively in an ERb template today and I figured I’d DRY it up by making a helper. Specifically, I was trying to output some given HTML unless a condition was met that should generate a hidden field instead—this case calls for handcrafted HTML generation.

I was doing things like:

<% ignore_these_fields = ['foo', 'bar'] %>

<% unless ignore_these_fields.include?('abc') %>
  <input type="text" name="abc" value="<%= @vals['abc'] %>" />
<% else %>
  <input type="hidden" name="abc" value="<%= @vals['abc'] %>" />
<% end %>

<% unless ignore_these_fields.include?('foo') %>
  <input type="radio" name="foo" value="<%= @vals['foo'] %>" /> One
  <input type="radio" name="foo" value="<%= @vals['foo'] %>" /> Two
<% else %>
  <input type="hidden" name="foo" value="<%= @vals['foo'] %>" />
<% end %>

<% unless ignore_these_fields.include?('bar') %>
  <select name="bar">
    <option <% 'selected' if @vals['bar']=='a'%>>blah1</option>
    <option <% 'selected' if @vals['bar']=='b'%>>blah2</option>
    <option <% 'selected' if @vals['bar']=='c'%>>blah3</option>
  </select>
<% else %>
  <input type="hidden" name="foo" value="<%= @vals['bar'] %>" />
<% end %>

Here, fairly different HTML form fields get rendered for each type of fields, and in some cases we do not want to show certain fields. So I wanted to do this instead: write a helper that would take a block and render its contents verbatim, otherwise create a hidden field.

<% ignore_these_fields = ['foo', 'bar'] %>

<% render_unless_ignored(ignore_these_fields, 'abc') do %>
  <input type="text" name="abc" value="<%= @vals['abc'] %>" />
<% end %>

<% render_unless_ignored(ignore_these_fields, 'foo') do %>
  <input type="radio" name="foo" value="<%= @vals['foo'] %>" /> One
  <input type="radio" name="foo" value="<%= @vals['foo'] %>" /> Two
<% end %>

<% render_unless_ignored(ignore_these_fields, 'bar') do %>
  <select name="bar">
    <option <% 'selected' if @vals['bar']=='a'%>>blah1</option>
    <option <% 'selected' if @vals['bar']=='b'%>>blah2</option>
    <option <% 'selected' if @vals['bar']=='c'%>>blah3</option>
  </select>
<% end %>

That’s a bit DRYer. But here came the tricky part: the helper. I had done what I thought was right:

def render_unless_ignored (ignoring_array, key)
  unless ignoring_array.include?(key)
    yield
  else
    %Q{<input type="hidden" name="#{CGI::escapeHTML key}" value="#{CGI::escapeHTML @vals[key]}" />}.html_safe
  end
end

The yielded content was showing up but the string result was not! The ERb template was not outputting it. Then a colleague pointed out that in this case we need to tell the ERb template to render the string with concat():

def render_unless_ignored (ignoring_array, key)
  unless ignoring_array.include?(key)
    yield
  else
    concat %Q{<input type="hidden" name="#{CGI::escapeHTML key}" value="#{CGI::escapeHTML @vals[key]}" />}.html_safe
  end
end

And THAT worked!