μLithp reader

μLithp - a Lisp in 27 lines of Ruby source

Primordial ooze | A reader | REPL | A compiler | Ruby interop

Extending μLithp with a reader

You can, if you choose, write μLithp code directly using Ruby arrays and symbols. However, writing a reader that reads in a more traditional textual Lisp-like syntax and converts it into the Ruby data structures. My friend Russ Olsen wrote a little reader, that I present below:

Reader

class Reader
  def initialize(expression)
    @tokens = expression.scan /[()]|\w+|".*?"|'.*?'/
  end

  def peek
    @tokens.first
  end

  def next_token
    @tokens.shift
  end

  def read
    return :"no more forms" if @tokens.empty?

    if (token = next_token) == '('
      read_list
    elsif token =~ /['"].*/
      token[1..-2]
    elsif token =~ /\d+/
      token.to_i
    else
      token.to_sym
    end
  end

  def read_list
    list = []
    list << read until peek == ')'
    next_token
    list
  end
end

Usage

Using the Reader class is simple as the following:

require 'lithp'
require 'reader'

Reader.new("(quote (1 2 3))").read
#=> [:quote, [1, 2, 3]]

l.eval Reader.new("(quote (1 2 3))").read
#=> [1, 2, 3]

And that's it.