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Use the Gemspec

Written by Magnus Holm

You can say a lot about GitHub’s previous gem hosting, but at least it helped us understanding one thing: the value of a gemspec. The gemspec is the README of the bits and the bytes of your code. It can both be understood and changed by both computers and humans. We shouldn’t fear the gemspec; we should embrace it.

Storing the data in the Rakefile and generating the gemspec seems totally backwards to me. Rakefile is for tasks, let’s place the data where it belongs: in the gemspec. Instead of building tools which generates the gemspec, let’s build tools that uses the gemspec.

Like gem build rails.gemspec.

Like the newest Gemify:

$ gemify
Currently editing gemify.gemspec

Which task would you like to invoke?
1) Change name (required) = gemify
2) Change summary (required) = The lightweight gemspec editor
3) Change version (required) = 0.3
4) Change author = Magnus Holm
5) Change email =
6) Change homepage =
7) Set dependencies

s) Save
r) Reload (discard unsaved changes)
m) Rename
l) List files

x) Exit

It’s not a solved problem though.

Rubygems can read any gemspec with Gem::Specification.load, but can only write it back again with #to_ruby in a normalized version.

Let’s have a look at Bundler’s excellent gemspec:

# -*- encoding: utf-8 -*- lib = File.expand_path('../lib/', __FILE__) $:.unshift lib unless $:.include?(lib) require 'bundler/version' do |s| = "bundler" s.version = Bundler::VERSION s.platform = Gem::Platform::RUBY s.authors = ["Carl Lerche", "Yehuda Katz", "André Arko"] = [""] s.homepage = "" s.summary = "The best way to manage your application's dependencies" s.required_rubygems_version = ">= 1.3.6" s.rubyforge_project = "bundler" s.add_development_dependency "rspec" s.files = Dir.glob("{bin,lib}/**/*") + %w(LICENSE s.executables = ['bundle'] s.require_path = 'lib' end

Short and concise. After running it through Gem::Specification.load(file) to_ruby:

# -*- encoding: utf-8 -*- do |s| = %q{bundler} s.version = "0.10.pre" s.required_rubygems_version =">= 1.3.6") if s.respond_to? :required_rubygems_version= s.authors = ["Carl Lerche", "Yehuda Katz", "Andr\303\251 Arko"] = %q{2010-04-03} s.default_executable = %q{bundle} = [""] s.executables = ["bundle"] s.files = %w[one huge array] s.homepage = %q{} s.require_paths = ["lib"] s.rubyforge_project = %q{bundler} s.rubygems_version = %q{1.3.6} s.summary = %q{The best way to manage your application's dependencies} if s.respond_to? :specification_version then current_version = Gem::Specification::CURRENT_SPECIFICATION_VERSION s.specification_version = 3 if >='1.2.0') then s.add_development_dependency(%q<rspec>, [">= 0"]) else s.add_dependency(%q<rspec>, [">= 0"]) end else s.add_dependency(%q<rspec>, [">= 0"]) end end

Boom, there goes your nice VERSION constant. And no more Dir globbing for you.

We need to make it possible to write interactive gemspec (heck, that’s why they are written in Ruby!) and make them easily readable by computers and make them easily modified by computers without losing the interactivity.