Peter Lewerin (2004-01-26): HereTcl is an unorthodox, radical, contrastandard, and hopefully interesting variant of Tcl...
> is the interpreter prompt // means to clear the results stack ; means the operation can't pop from input
(1) Values are simply pushed on the results stack until an
operator appears
> 2 3 4
4 3 2
(2a) Postfix: two arguments are popped from the results stack
> // 2 3 4 + > // 2 3 4 -
7 2 -1 2
(2b) Infix: one argument is popped from the results stack; the
other is read from the token stream
> // 2 3 + 4 > // 2 3 - 4
7 2 -1 2
(3a) That is, unless the next token is ";" in which case
the interpreter must attempt to evaluate as postfix
> // 2 3 + ; 4 > // 2 3 - ; 4
4 5 4 -1
(3b) The above is equivalent to
> // 2 3 + > // 2 3 -
5 -1
> 4 > 4
4 5 4 5
(3c)
> // 2 + 3 4 > // 2 - 3 4
4 5 4 -1
(4) There must be at least one value on the results
stack before the operator is read
> // + 2 3 4
error: addition requires two terms
(5) But the values on the results stack can be left
over from previous operations
> // 2
2
> + 3 4
4 5
> + 3 4
4 7 5RS: so binary operators work either infix or postfix (RPN), and the stack is returned, top at left?
(PL): Yes. The stack direction is simply the result of dumping a tcllib stack.
Stay tuned for more interesting examples...