This may seem like a contrived example but it comes from the moustache compatibility tests in Handlebars.js' acceptance suite.

impl.rb

require 'flavour_saver'

# User's helper code:

FS.register_helper(:link) do |prefix,&block|
  "<a href='#{prefix}/#{url}'>#{text}</a>"
end

Link = Struct.new(:url, :text)
Context = Struct.new(:prefix, :links)

FS.evaluate(File.read("template.handlebars"), Context.new('foo', [Link.new("abc", "A B C"), Link.new("def", "D E F"))

# Should return:
# <a href='foo/abc'>A B C</a>
# <a href='foo/def'>D E F</a>
# But doesn't because #url and #text NameError inside the block
# is not evaluated inside the correct context. Complication is
# that the helper block can optionally take a block (ie it's a
# handlebars block helper.

template.handlebars

{{#each links}}
  {{link ../prefix}}
{{/each}}