Ruby templates: How to pass variables into inlined ERB?

Question

I have an ERB template inlined into Ruby code:

require \'erb\'

DATA = {
    :a => \"HELLO\",
    :b => \"WORLD\",
}

template = ERB.new <<-EOF
          


        

Answer 1

Maybe the cleanest solution would be to pass specific current local variable to erb template instead of passing the entire binding. It's possible with ERB#result_with_hash method (introduced in Ruby 2.5)

DATA.keys.each do |current|
  result = template.result_with_hash(current: current)
...

Answer 2

For a simple solution, use OpenStruct:

require 'erb'
require 'ostruct'
namespace = OpenStruct.new(name: 'Joan', last: 'Maragall')
template = 'Name: <%= name %> <%= last %>'
result = ERB.new(template).result(namespace.instance_eval { binding })
#=> Name: Joan Maragall

The code above is simple enough but has (at least) two problems: 1) Since it relies on OpenStruct, an access to a non-existing variable returns nil while you'd probably prefer that it failed noisily. 2) binding is called within a block, that's it, in a closure, so it includes all the local variables in the scope (in fact, these variables will shadow the attributes of the struct!).

So here is another solution, more verbose but without any of these problems:

class Namespace
  def initialize(hash)
    hash.each do |key, value|
      singleton_class.send(:define_method, key) { value }
    end 
  end

  def get_binding
    binding
  end
end

template = 'Name: <%= name %> <%= last %>'
ns = Namespace.new(name: 'Joan', last: 'Maragall')
ERB.new(template).result(ns.get_binding)
#=> Name: Joan Maragall

Of course, if you are going to use this often, make sure you create a String#erb extension that allows you to write something like "x=<%= x %>, y=<%= y %>".erb(x: 1, y: 2).

Answer 3

EDIT: This is a dirty workaround. Please see my other answer.

It's totally strange, but adding

current = ""

before the "for-each" loop fixes the problem.

God bless scripting languages and their "language features"...

Answer 4

Simple solution using Binding:

b = binding
b.local_variable_set(:a, 'a')
b.local_variable_set(:b, 'b')
ERB.new(template).result(b)

Answer 5

require 'erb'

class ERBContext
  def initialize(hash)
    hash.each_pair do |key, value|
      instance_variable_set('@' + key.to_s, value)
    end
  end

  def get_binding
    binding
  end
end

class String
  def erb(assigns={})
    ERB.new(self).result(ERBContext.new(assigns).get_binding)
  end
end

REF : http://stoneship.org/essays/erb-and-the-context-object/

Answer 6

I can't give you a very good answer as to why this is happening because I'm not 100% sure how ERB works, but just looking at the ERB RDocs, it says that you need a binding which is "a Binding or Proc object which is used to set the context of code evaluation".

Trying your above code again and just replacing

result = template.result

with

result = template.result(binding)

made it work.

I'm sure/hope someone will jump in here and provide a more detailed explanation of what's going on. Cheers.

EDIT: For some more information on Binding and making all of this a little clearer (at least for me), check out the Binding RDoc.