Update README.md

README.md

@@ -17,6 +17,109 @@ The syntax is a superset of the Forth language. In FCL there are literal syntax
 
 ## Control structures
 
+FCL supports the traditional Forth conditional and loop control structures.
+
+General form of `if else then`.
+
+```forth
+<bool> if <consequent> else <alternative> then
+```
+
+For example:
+```forth
+: max ( a b -- max )
+  2dup < if nip else drop then ;
+
+10 100 max . \ prints 100
+```
+
+The else part is optional.
+
+```forth
+: abs ( n -- absn )
+  dup 0< if -1 * then ;
+
+-10 abs . \ prints 10
+```
+
+#### Case statement
+
+FCL supports switch-case like flow control logic as shown in the following example.
+
+```forth
+: day ( n -- )
+  case
+    1 of print: 'Monday' endof
+    2 of print: 'Tuesday' endof
+    3 of print: 'Wednesday' endof
+    4 of print: 'Thursday' endof
+    5 of print: 'Friday' endof
+    6 of print: 'Saturday' endof
+    7 of print: 'Sunday' endof
+    drop 'Unknown'
+  endcase ;
+````
+
+#### Count-controlled loops
+
+The `limit` and `start` before the word `do` defines the number of times the loop will run.
+
+```forth
+<limit> <start> do <loop-body> loop
+```
+
+*Do* loops iterate through integers by starting at *start* and incrementing until you reach the *limit*. The word *i* pushes the loop index onto the stack. In a nested loop, the inner loop may access the loop variable of the outer loop by using the word *j*.
+
+For example:
+```forth
+5 0 do i . loop \ prints 0 1 2 3 4
+```
+
+It is important to understand the implementation details of this loop. `DO` loops store the loop index on the return stack. You can break the semantics of *i* and *j* if you use the return stack to store temporary data. Exiting from the loop requires clearing up the return stack by using the `unloop` word.
+
+#### Condition-controlled loops
+
+##### until loop
+
+```forth
+begin <loop-body> <bool> until
+```
+The *begin*...*until* loop repeats until a condition is true. This loop always executes at least one time.
+
+For example:
+
+```forth
+: countdown ( n -- )
+  begin
+    dup .
+    1- dup
+  0 < until
+  drop ;
+
+5 countdown \ prints 5 4 3 2 1 0
+```
+
+##### while loop
+
+```forth
+begin .. <bool> while <loop-body> repeat
+```
+For example:
+```forth
+: countdown ( n -- )
+  begin
+    dup 0 >=
+  while
+    dup . 1-
+  repeat
+  drop ;
+
+5 countdown \ prints 5 4 3 2 1 0
+```
+
+Control structres are compile time words with no interpretation semantics.
+
+
 ## Locals
 
 ## Maps