Create bundlerApp drvs with ease.

make-bundler-app.rb

#! /usr/bin/env nix-shell
#! nix-shell -i ruby -p ruby bundix bundler

require 'optparse'
require 'rubygems/package'
require 'rubygems/remote_fetcher'
require 'erb'

op = OptionParser.new do |o|
  o.banner = "Usage: make-bundler-app.rb GEM_NAME [options]"
  o.on('-h', '--help', 'Prints this help'){ puts o; exit }
  o.on('--licenses a,b', Array)
  o.on('--maintainers a,b', Array)
  o.on('--homepage URL')
  o.on('--description STRING')
  o.on('--version STRING')
end

options = {
  description: nil,
  licenses: [],
  maintainers: [],
  homepage: nil,
  version: nil,
}

op.parse!(into: options)

gem = ARGV[0]

dependency = Gem::Dependency.new(gem, options[:version])
gem_file = Gem::RemoteFetcher.fetcher.download_to_cache(dependency)
package = Gem::Package.new(gem_file)
options[:spec] = package.spec

default_nix = ERB.new(<<EOF, nil, '-').result_with_hash(options)
{ lib, bundlerApp }:

bundlerApp {
  pname = "<%= spec.name %>";
  gemdir = ./.;
  exes = [ <%= spec.executables.map{ |e| e.dump }.join(' ') %> ];

  meta = with lib; {
    description = "<%= (description || spec.description).to_s.gsub(/\.$/, '') %>";
    homepage = <%= homepage || spec.homepage %>;
<% if licenses.size == 1 -%>
    license = licenses.<%= licenses.first %>;
<% elsif licenses.size > 1 -%>
    license = with licenses; [ <%= licenses.join(' ') %> ];
<% end -%>
<% if maintainers.size > 0 -%>
    maintainers = with maintainers; [ <%= maintainers.join(' ') %> ];
<% end -%>
  };
}
EOF

FileUtils.mkdir_p(gem)
Dir.chdir gem do
  File.write('default.nix', default_nix)

  File.write('Gemfile', <<~EOF)
    source 'https://rubygems.org' do
      gem #{gem.dump}, #{dependency.requirement.to_s.dump}
    end
  EOF

  puts 'generating lockfiles...'
  system('bundle', 'lock')
  system('bundix')
end