Implement map

lib/chervil/core.rb

       end
     end
 
+    def self.eval(source, env)
+      lexer = Lexer.new(source)
+      tree = Parser.new(lexer).parse
+
+      if tree.is_a?(Error)
+        return tree
+      end
+
+      interpreter = Interpreter.new(tree, env)
+      interpreter.interpret.first
+    end
+
     CORE = {
       "+" => Proc.new { |args| type_check(args, Float) || args.inject(:+) },
       "-" => Proc.new { |args| type_check(args, Float) || args.inject(:-) },
             els.prepend(el)
           end
         end
-      end
+      end,
+    }
+
+    NATIVE_CORE = {
+      "map" => "(define (map f xs) (if (= xs '()) '() (cons (f (car xs)) (map f (cdr xs)))))"
     }
   end
 end

lib/chervil/env.rb

         set(k, v)
       end
 
+      Core::NATIVE_CORE.each do |k, v|
+        Core.eval(v, self)
+      end
+
       unless parent.nil?
         parent.data.each do |k, v|
           set(k, v)

spec/core_spec.rb

         [1.0, 2.0, 3.0]
       )
     end
+
+    it 'implements map' do
+      env = Env.new
+      Core.eval("(define (plus-one x) (+ x 1))", env)
+      expect(Core.eval("(map plus-one '(1 2 3))", env)).to eq(
+        [2.0, 3.0, 4.0]
+      )
+    end
   end
 end