Email template system for constructing plaintext emails (the example shows usage in a Sinatra route, with the mail gem)

emailtemplate.rb

class EmailTemplate
    def initialize
        @io = StringIO.new
    end
    def puts(*args)
        @io.puts *args
    end
    def print(*args)
        @io.print *args
    end
    def render(*args, &block)
        # this loop is to catch next/break from the block, iirc
        1.times do
            # this lets us override puts and print in the block
            instance_exec *args, &block
        end
        @io.string.gsub(/\r/, "")
    end
    def self.render(*args, &block)
        EmailTemplate.new.render *args, &block
    end
end

samplemail.rb

# this gets eval()'d in the context of the Sinatra route, so instance vars (@u) work
# (but it must be passed as an argument to the template block)
EmailTemplate.render(@u) do |u|
    if u.balance >= 0
        puts "(no debt)"
        next
    end

    puts <<EOF
Howdy! blah blah..
Multiple lines too.
EOF

    if u.stuff?
        puts "Also, stuff."
    end

    puts "\n\n"
end

sendemail.rb

post "/email" do
    m = Mail.new do
        from    "me@here.com"
        subject "Whatever"
        to      @u.email
    end

    m.body = eval(File.read("samplemail.rb"), binding, "samplemail.rb")
    m.charset = "UTF-8"

    m.deliver

    haml "%h2 Message sent!\n%p To: #{m.to}"
end