closure tests and fix

docs/advanced_topics.md

@@ -0,0 +1,50 @@
+# Advanced topics
+
+## Closures/scopes isolation
+
+Each block has own scope, in which it can safely uses closures and override
+outer vars:
+
+    var param = "global"
+    val prefix = "param in "
+    val scope1 = {
+        var param = prefix + "scope1"
+        param
+    }
+    val scope2 = {
+        var param = prefix + "scope2"
+        param
+    }
+    // note that block returns its last value
+    println(scope1)
+    println(scope2)
+    println(param)
+    >>> param in scope1
+    >>> param in scope2
+    >>> global
+    >>> void
+
+One interesting way of using closure isolation is to keep state of the functions:
+
+    val getAndIncrement = {
+        // will be updated by doIt()
+        var counter = 0
+
+        // we return callable fn from the block:
+        fun doit() {
+            val was = counter
+            counter = counter + 1
+            was
+        }
+    }
+    println(getAndIncrement())
+    println(getAndIncrement())
+    println(getAndIncrement())
+    >>> 0
+    >>> 1
+    >>> 2
+    >>> void
+
+Inner `counter` is not accessible from outside, no way; still it is kept 
+between calls in the closure, as inner function `doit`, returned from the
+block, keeps reference to it and keeps it alive.