Updated Application overview (markdown)

Application-overview.md

@@ -224,3 +224,59 @@ RootNode 1:16
             └─BoolLiteralNode 1:37
                └ (bool, »false«, 1:37)
 ```
+
+All parsers-related code is located in `io.smnp.dsl.ast` package.
+
+## Evaluator
+Evaluator is the last stage of SMNP language processing pipeline and also is the heart
+of entire SMNP tool, which takes AST as an input and performs programmed operations.
+Similar to implemented parser, evaluator works *recursively* because of processing tree-like
+structure.
+Evaluator's architecture is similar to parser's one. Evaluator consists of
+smaller evaluators which are able to evaluate small part of AST's node types.
+Similar to parsers, the evaluators also uses a helper method (like `oneOf()`) to improve readability and decrease 
+the complexity along with the code repeatability.
+
+Because evaluator introduces as *runtime* term, it also works on special object
+called *environment*. The *environment* object contains some runtime information,
+like loaded modules (with included functions and methods), call stack with included scopes and some meta information. This object is passed through all evaluators along with AST and its
+subtrees.
+
+Following listening shows the example evaluator which is `if` statement evaluator:
+```kotlin
+class ConditionEvaluator : Evaluator() {
+   private val expressionEvaluator = ExpressionEvaluator()
+   private val defaultEvaluator = DefaultEvaluator()
+
+   override fun supportedNodes() = listOf(ConditionNode::class)
+
+   override fun tryToEvaluate(node: Node, environment: Environment): EvaluatorOutput {
+      val (conditionNode, trueBranchNode, falseBranchNode) = (node as ConditionNode)
+      val condition = expressionEvaluator.evaluate(conditionNode, environment).value
+
+      if (condition.type != DataType.BOOL) {
+         throw contextEvaluationException(
+            "Condition should be of bool type, found '${condition.value}'",
+            conditionNode.position,
+            environment
+         )
+      }
+
+      if (condition.value as Boolean) {
+         return defaultEvaluator.evaluate(trueBranchNode, environment)
+      } else if (falseBranchNode !is NoneNode) {
+         return defaultEvaluator.evaluate(falseBranchNode, environment)
+      }
+
+      return EvaluatorOutput.ok()
+   }
+}
+```
+The code above defines list of supported node, which in this case is a list with single element: `ConditionNode`. The `ConditionNode` is product of `ConditionParser`'s work which handles the `if` statements.
+The `tryToEvaluate()` method contains the actually logic of evaluation, and in this case it:
+1. evaluates the condition using `ExpressionEvaluator` (it always returns a value)
+2. asserts the value to be of `bool` type - if it's other than `bool`, an exception is begin thrown 
+3. evaluates the `trueBranchNode` if the value is evaluated to `true`
+4. if the condition is evaluated to `false`, it checks if `falseBranchNode` (that comes from `else` clause) is present. If so, it's being evaluated.
+
+All evaluator-related code is located in `io.smnp.evaluation` package.