Use a here string and lambda in a Sinatra helper method - ruby

Question

I'm trying to create a Sinatra helper that returns a dynamically-generated HTML. I thought I would use a here string for the static bit and a lambda to calculate the dynamic part.
foo_helper.rb:
require 'erb'
module FooHelper
def tabs(selected)
template = ERB.new <<~HERE
<ul class="nav nav-tabs">
<li class="nav-item"><a class="nav-link <%= 'active' if selected == 'favorites' %>" href="/foo/favorites">Favorites</a></li>
<li class="nav-item"><a class="nav-link <%= 'active' if selected == 'all' %>" href="/foo">All</a></li>
<%= alpha.call %>
</ul>
HERE
# binding to a string works as expected
# alpha = "<li class='nav-item'><a class='nav-link' href='/foo/a'>A</a></li>"
# binding to a lambda, doesn't
alpha = lambda {
('a'..'z').each do |letter|
"<li class='nav-item'><a class='nav-link #{ 'active' if selected == letter }' href='/foo/#{letter}'>#{letter}</a></li>"
end
}
template.result(binding)
end
end
foo_controller.rb:
class FooController < ApplicationController
helpers FooHelper if defined?(FooHelper)
...
end
index.erb:
...
<%= tabs('favorites') %>
...
Results:
Displays the range, rather than the individual lis.
Am I missing something in the lambda?
** edit **
Corrected the numerous errors.

Answer 1

There are just too many mistakes in your code.
#nav is defined at module level, but accessed at instance level, so you got nil when you need it.
When you call a lambda, you need a dot between the variable name and the opening parenthesis, like foo.(123)
#nav.foo(binding), really? What's the (supposed) type of #nav? Does that type have the instance method foo?
<%= foo %> won't execute foo, because it's a local variable, not a method.

Answer 2

The reason why it renders a..z is because the Range#each method (called from within the lambda) executes a given block for each element and then returns the range itself again.
What you would want to use here instead is the Enumerable#map method. Similar to #each it also executes a block for each element, but the return values of said block are returned in a new array.
For comparison:
p ("a".."c").each { |x| x.upcase }
#=> "a".."c"
p ("a".."c").map { |x| x.upcase }
#=> ["A", "B", "C"]