[words] add SYSEVAL to run underlying OS commands

lib/rpl.rb

@@ -41,7 +41,7 @@ class Rpl < Interpreter
   end
 
   def run!( input )
-    stack = super
+    stack = super( input )
 
     persist_state if @live_persistence
 
@@ -64,6 +64,7 @@ class Rpl < Interpreter
   prepend RplLang::Words::Store
   prepend RplLang::Words::String
   prepend RplLang::Words::StringAndList
+  prepend RplLang::Words::System
   prepend RplLang::Words::Test
   prepend RplLang::Words::TimeAndDate
   prepend RplLang::Words::Trig

lib/rpl/words.rb

@@ -18,4 +18,5 @@ require 'rpl/words/time-date'
 require 'rpl/words/trig'
 require 'rpl/words/logarithm'
 require 'rpl/words/filesystem'
+require 'rpl/words/system'
 require 'rpl/words/list'

lib/rpl/words/system.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module RplLang
+  module Words
+    module System
+      include Types
+
+      def populate_dictionary
+        super
+
+        category = 'System'
+
+        @dictionary.add_word!( ['syseval'],
+                               category,
+                               '( string -- output ) run string in OS shell and put output on stack',
+                               proc do
+                                 args = stack_extract( [[RplString]] )
+
+                                 @stack << Types.new_object( RplString, "\"#{`#{args[0].value}`}\"" )
+                               end )
+      end
+    end
+  end
+end