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
    current key is: <%= current %>
    current value is: <%= DATA[current] %>
EOF

DATA.keys.each do |current|
    result = template.result
    outputFile = File.new(current.to_s,File::CREAT|File::TRUNC|File::RDWR)
    outputFile.write(result)
    outputFile.close
end

I can't pass the variable "current" into the template.

The error is:

(erb):1: undefined local variable or method `current' for main:Object (NameError)

How do I fix this?

Answer 1

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).