Maybe something like this?

```ruby
set :procfile_contents, -> {
  contents = ''  
  contents << "web: "
  contents << "chruby-exec #{fetch(:chruby_ruby)} -- "
  contents << "bundle exec "
  contents << "thin start "
  contents << "-S /tmp/#{fetch(:app_name)}.thin-1.sock "
  contents << "-e #{fetch(:stage)}"
}
```

Inspiration from the next to last bullet point at https://github.com/bbatsov/ruby-style-guide#strings.