Adding reading method

Add:  	- Reading null variables and instances
	- Using step interpreter to interpretation


Signed-off-by: Florian Deljarry <[email protected]>

src/interpreter/interpreter.nit

@@ -15,5 +15,5 @@
 # Interpretation of Nit programs
 module interpreter
 
-import naive_interpreter
+import step_interpreter
 import dynamic_loading_ffi

src/interpreter/naive_interpreter.nit

@@ -446,13 +446,33 @@ class NaiveInterpreter
 		return f.map[v]
 	end
 
+	# Retrieve the value of the variable in the current frame
+	fun read_null_variable(v: Variable):nullable Instance
+	do
+		var f = frames.first.as(InterpreterFrame)
+		if f.map.has_key(v) then return f.map[v]
+		return null
+	end
+
 	# Assign the value of the variable in the current frame
 	fun write_variable(v: Variable, value: Instance)
 	do
 		var f = frames.first.as(InterpreterFrame)
 		f.map[v] = value
 	end
 
+	# Retrieve the variable of the Instance in the current frame
+	fun read_instance(i: nullable Instance): nullable Variable
+	do
+		if i != null then
+			var f = frames.first.as(InterpreterFrame)
+			for key, value in f.map do
+				if value == i then return key
+			end
+		end
+		return null
+	end
+
 	# Store known methods, used to trace methods as they are reached
 	var discover_call_trace: Set[MMethodDef] = new HashSet[MMethodDef]