Last result local '__' (#63)

Inspired by https://github.com/pry/pry/wiki/Special-Locals#Last_result

spec/integration/icr_spec.cr

@@ -301,4 +301,56 @@ describe "icr command" do
     CRYSTAL
     icr(input).should match /42/
   end
+
+  context "__" do
+    it "returns last value" do
+      icr("a = 42\n__").should match /42/
+    end
+
+    it "returns nil if there is no last value" do
+      icr("__").should match /nil/
+    end
+
+    context "in expressions" do
+      it "works with unary operators" do
+        icr("true\n!__").should match /false/
+      end
+
+      it "works with binary operators" do
+        icr("42\n__ + 1").should match /43/
+        icr("42\n1 + __").should match /43/
+      end
+
+      it "allows method calls" do
+        input = <<-CRYSTAL
+          "aabbcc"
+          __.count('a')
+        CRYSTAL
+        icr(input).should match /2/
+      end
+
+      it "works in methods/blocks" do
+        input = <<-CRYSTAL
+          38
+          def add(v)
+            __ + v
+          end
+
+          add(3)
+
+          -> { __ + 1 }.call
+        CRYSTAL
+
+        icr(input).should match /42/
+      end
+    end
+
+    it "is not interpreted as the last value if is a part of var name or literal" do
+      icr("__v = 0").should match /0/
+      icr("v__ = 0").should match /0/
+      icr("v__1 = 0").should match /0/
+      icr("filename = \"spec__helper.cr\"").should match /spec__helper.cr/
+      icr("require \"secure_random\"").should match /ok/
+    end
+  end
 end

src/icr/console.cr

@@ -35,11 +35,11 @@ module Icr
       end
     end
 
-    private def paste_mode()
+    private def paste_mode
       puts "# Entering paste mode (ctrl-D to finish)"
       input = String.build do |input|
         loop do
-          input_line = Readline.readline()
+          input_line = Readline.readline
 
           if input_line.nil?
             puts "\n\n# Ctrl-D was pressed, exiting paste mode...\n"
@@ -58,7 +58,12 @@ module Icr
       end
     end
 
+    private def last_value
+      @executer.execute.value
+    end
+
     private def process_command(command : String)
+      command = command.to_s.gsub(/\b__\b/) { last_value.to_s.strip }
       result = check_syntax(command)
       process_result(result, command)
     end