He explains that API design is very similar to language design, and I immediately thought that this particularly holds for APIs which are designed as domain-specific languages, which is one of Ruby’s strengths (read this article from Oreillynet.com to learn more about DSLs).

A few slides later he talks about characteristics of good APIs:

• Easy to learn
• Easy to use, even without documentation
• Hard to misuse
• Easy to read and maintain code that uses it
• Sufficiently powerful to satisfy requirements
• Easy to evolve
• Appropriate to audience

To achieve all these properties for an API is certainly non-trivial, but at least some of them get easier if you design APIs as a domain-specific languages: As the term says, a DSL provides a language that is tailored for a particular domain, in addition DSLs are generally declarative and hide language aspects that are irrelevant. Such a language is obviously easier to learn, use and to read than a mere API, and is more appropriate to its audience. In the end, these are known strengths of declarative languages. Here’s an example taken from the Capistrano manual (Capistrano is a Ruby DSL for remote software deployment):
[ruby]
desc “Start the spinner daemon”
task :spinner, :roles => :app do
run “#{current_path}/script/spin”
end

desc “Used only for deploying when the spinner isn’t running”
task :cold_deploy do
transaction do
update_code
symlink
end

spinner
end
[/ruby]
Though this doesn’t hide imperative aspects completely, I think it’s a good example of how a DSL feels – just as if it wasn’t Ruby but a different language.

So, is it safe to conclude that DSLs are the better APIs?