Use env in interpreter

lib/chervil/interpreter.rb

 module Chervil
   class Interpreter
-    def initialize(tree)
+    def initialize(tree, env = Env.new)
       @tree = tree
+      @env = env
     end
 
     def interpret
-      env = Env.new
-      @tree.map { |node| node.evaluate(env) }
+      @tree.map { |node| node.evaluate(@env) }
     end
   end
 end

lib/chervil/parser.rb

         eat(:identifier)
         name = identifier
         value = expr
-        p @current_token
         eat(:rparen)
         return AST::Definition.new(name, value)
       end

spec/interpreter_spec.rb

+module Chervil
+  RSpec.describe Interpreter do
+    def interpret(source, env)
+      lexer = Lexer.new(source)
+      tree = Parser.new(lexer).parse
+      Interpreter.new(tree, env).interpret
+    end
+
+    it 'interprets a definition and variable usage' do
+      env = Env.new
+      interpret('(define x 5)', env)
+      expect(env.get("x")).to eq(AST::Number.new(5.0))
+      expect(interpret('x', env).first).to eq(AST::Number.new(5.0))
+    end
+  end
+end