Add tasks for all the new languages

Task/Parsing-RPN-to-infix-conversion/EchoLisp/parsing-rpn-to-infix-conversion.echolisp

@@ -0,0 +1,60 @@
+(require 'hash)
+(string-delimiter "")
+
+(define (^ a b ) (expt a b)) ;; add this not-native function
+(define-syntax-rule (r-assoc? op) (= op  "^"))
+(define-syntax-rule (l-assoc? op) (not ( = op "^" )))
+(define PRECEDENCES (list->hash
+	'(("+" . 2) ("-" . 2) ("*" . 3) ("/" . 3) ("//" . 3) ("^" . 4))
+	(make-hash)))
+
+;; RPN vector or list -> infix tree  ->  (a op (b op c) d) ..
+(define (rpn->infix rpn)
+(define S (stack 'S))
+	(for ((token rpn))
+	(if (procedure? token)
+		(let [(op2 (pop S)) (op1 (pop S))]
+			(unless (and op1 op2)   (error "cannot translate expression" rpn))
+	    	(push S (list op1 token op2))
+	    	)
+	    (push S token ))
+	(writeln 'token (string token) 'stack (stack->list S)))
+	(begin0
+		(pop S) ;; return (top S)
+		(unless (stack-empty? S) (error "ill-formed rpn" rpn)))
+	)
+	
+;; a node  tree is (left op right) or a  number
+(define-syntax-id _.left (first _)) ; mynode.left expands to (first mynode)
+(define-syntax-id _.right (third _))
+(define-syntax-id _.op  (string (second _ )))
+(define-syntax-rule (precedence node) (hash-ref PRECEDENCES (string (second node))))
+
+(define (left-par? node) ; does lhs needs ( lhs ) ?
+	(cond
+	[(number? node.left) #f]
+	[(< (precedence node.left) (precedence node)) #t]
+	[(and
+		 	(r-assoc? node.op)
+		 	(= (precedence node.left) (precedence node))) #t]
+	[else #f]))
+	
+(define (right-par? node)
+	(cond
+	[(number? node.right) #f]
+	[(< (precedence node.right) (precedence node)) #t]
+	[(and
+		 	(l-assoc? node.op)
+		 	(= (precedence node.right) (precedence node))) #t]
+	[else #f]))
+	
+;; infix tree -> char string
+(define (infix->string node)
+	(cond
+	[(number? node) (string node)]
+	[else (let
+		[(lhs (infix->string node.left))
+		(rhs (infix->string node.right))]
+		(when (left-par? node) (set! lhs (string-append "(" lhs ")")))
+		(when (right-par? node) (set! rhs (string-append "(" rhs ")")))
+		(string-append lhs " " node.op " " rhs))]))