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NaturalDSL

An experimental (and highly likely useless for real–world) DSL to build a natural–ish DSL language and write your programs using it. Right to the example:

lang = NaturalDSL::Lang.define do
  command :route do
    keyword :from
    token
    keyword :to
    token
    keyword(:takes).with_value

    execute do |vm, city1, city2, distance|
      distances = vm.read_variable(:distances) || {}
      distances[[city1, city2]] = distance
      vm.assign_variable(:distances, distances)
    end
  end

  command :how do
    keyword :long
    keyword :will
    keyword :it
    keyword :take
    keyword :to
    keyword :get
    keyword :from
    token
    keyword :to
    token

    execute do |vm, city1, city2|
      distances = vm.read_variable(:distances) || {}
      distance = distances[[city1, city2]].value
      "Travel from #{city1.name} to #{city2.name} takes #{distance} hours"
    end
  end
end

result = NaturalDSL::VM.run(lang) do
  route from london to glasgow takes 22
  route from paris to prague takes 12
  how long will it take to get from london to glasgow
end

puts result # => Travel from london to glasgow takes 22 hours

Read more about this experiment in my blog.

Language definition

Command syntax

Each language consists of commands. Command can contain keywords, tokens and values:

  • keyword is something you want to be in the command to be semantically correct, but you don't need to have it to execute the command (e.g., to, from, etc.);
  • token is anything that user types, and the typed word will be passed to the execution block;
  • value can be read right after the last keyword or token with with_value modifier (e.g., value 42).

For instance:

       keyword  token   value
       ↓        ↓       ↓
assign variable a value 1
↑                 ↑
command name      keyword

Command execution

Command makes no sense without logic it implements. We can configure it using the execute method: it receives the instance of the current Virtual Machine as well as all tokens and values:

execute do |vm, *args|
  # logic goes here
end

This is how we can create a very basic command that remembers values:

command :assign do
  keyword :variable
  token
  keyword(:value).with_value

  execute do |vm, token, value|
    # how to assign?
  end
end

Shared data

We need to store the data somewhere between commands, and Virtual Machine has that storage, which can be accessed using assign_variable and read_variable. Here is the whole definition of language that can store and sum variables:

lang = NaturalDSL::Lang.define do
  command :assign do
    keyword :variable
    token
    keyword(:value).with_value

    execute { |vm, token, value| vm.assign_variable(token, value) }
  end

  command :sum do
    token
    keyword :with
    token

    execute do |vm, left, right|
      vm.read_variable(left).value + vm.read_variable(right).value
    end
  end
end

Running languages

Finally, we can run the program written in our new DSL using the VM class:

NaturalDSL::VM.run(lang) do
  assign variable a value 1
  assign variable b value 2
  sum a with b
end

Multiple primitives

Need to consume the unknown amount of similar primitives? Use zero_or_more:

lang = NaturalDSL::Lang.define do
  command :expose do
    token.zero_or_more

    execute { |_, *fields| "exposing #{fields.join(', ')}" }
  end
end

result = NaturalDSL::VM.run(lang) do
  expose id email
end

puts result # => exposing id, email

Alternative name for #value

Sometimes you don't want to see the word value in your commands. In this case you can rename it by passing an argument:

lang = NaturalDSL::Lang.define do
  command :john do
    keyword(:takes).with_value
    execute { |vm, value| vm.assign_variable(:john, value) }
  end

  command :jane do
    keyword(:takes).with_value
    execute { |vm, value| vm.assign_variable(:jane, value) }
  end

  command :who do
    keyword :has
    keyword :more

    execute do |vm|
      name = %i[john jane].max_by { |person| vm.read_variable(person).value }
      "#{name} has more apples!"
    end
  end
end

result = NaturalDSL::VM.run(lang) do
  john takes 2
  jane takes 3
  who has more
end

puts result # => jane has more

Want some fun?

Here are a couple of ideas to work on 🙂

Optional parts

Let's allow parts that can be omitted:

lang = NaturalDSL::Lang.define do
  command :assign do
    keyword(:variable).optional
    token
    keyword(:value).with_value

    execute { |vm, token, value| vm.assign_variable(token, value) }
  end
end

result = NaturalDSL::VM.run(lang) do
  assign variable a value 1
  assign b value 2
end

Subcommands

What if I want to start two commands with the same word? Example:

lang = NaturalDSL::Lang.define do
  command :mov do
    option do
      token.with_value

      execute do |vm, register, value|
        # do constant assigment
      end
    end

    option do
      token
      token

      execute do |vm, register, register_with_value|
        # copy value from register
      end
    end
  end
end

NaturalDSL::VM.run(lang) do
  mov a 9
  mov a b
end

Installation

Add this line to your application's Gemfile, and you're all set:

gem "natural_dsl"

License

The gem is available as open source under the terms of the MIT License.

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