RPN-Lang

JavaScript implementation of an interpreter for a little language based on Reverse Polish Notation (RPN)

Try it out here!

Language

Evaluates a reverse polish notation expression against a set of variables. The expected format is that white-space separates each token. The basic run-time elements are the stack of parsed tokens and a hash-table of variables.

Constants

Token	Description
val	Any string that does not match an operation is interpreted as a literal value.
\val	A backslash is removed from the start of a constant and the rest of the token is always taken as a constant. Use this to escape an operator to be processed as a constant.
#	Pops and discards the last token on the stack, use along with { ... } to make comments
true	A boolean true value.
false	A boolean false value.
undef	The value a variable has before it is assigned to.
"	Push a single space onto the stack.
\n	Push a new line character onto the stack.
{ ... }	Quotes the enclosed tokens into a single token on the stack. Can be nested.
::	Clone the top token on the stack. Useful for when you want to calculate a result based on it but also keep it.
$	Pushes the current stack size (prior to pushing) onto the stack.

Boolean operators

Token	Description
==	Pop two values and push true if they are equal, else push false
!=	Pop two values and push false if they are equal, else push true
<	Pop two values and push whether the second one is less than the top: "6 3 <" is false; "3 6 <" is true
>	The following comparison operators all function similar to '<'
<=
>=
||	Pop two values and push the logical OR of them
&&	Pop two values and push the logical AND of them
!	Pop one value and push the logical NOT of them

Numerical operators

Token	Description
+	Pop two values and push their sum
-	Pop two values and subtract the first/top from the second: "3 4 -" is -1; "4 3 -" is 1
*	Pop two values and push their product
/	Pop two values and divide the second one by the first/top: "6 3 /" is 2; "3 6 /" is .5
\	Pop two values and do integer division (round down any fractional part): "6 3 " is 2; "3 6 " is 0
%	Pop two values and get the modulus value from division: "6 3 %" is 0; "3 6 %" is 3
&	Pop two values and push a bit-wise AND of them
|	Pop two values and push a bit-wise OR of them
^	Pop two values and push a bit-wise XOR of them
~	Pop one value and push the bit-wise NOT of it
<<	Pop two values and use the top one to shift the second one left (increase value).
>>	Pop two values and use the top one to shift the second one right (decrease value).

String operators

Token	Description
.	Concatenate top two values (as strings) on the stack and push the result.
[]	Break a token into a list of characters followed by a count. Each character and the count becomes a distinct token.

Control operators

Token	Description
?:	Ternary, or if-then-else, operator: "test a b ?:" evals to a if test is true and to b if test is false.
()	Pop the top of stack and evaluates it as a RPNLang expression. Typically, the top of stack should be a { ... } quoted expression.
<>	Has no effect on the program, but sets a breakpoint so if the interpreter is running in a debug mode it can choose to stop at this point.
!!	Flushes the stack to output, joins all current values without delimiters and adds a newline to the end. If the stack is empty, does nothing.

Variable operators

Token	Description
:=	This is the variable assignment operator: "5 a :=" sets a to 5. Previously assigned values aren't actually lost, each variable is actually a stack, and this pushes on a new value.
?	Evaluates the top of stack token as a variable name and pushes on its value in its place. This is a "peek" on the variable stack.
?!	Takes the top of stack token as a variable name and un-assigns the last assigned value. The previously assigned value of the variable will now be in place. The value that was un-assigned is pushed onto the stack. Use # if you want to ignore it. This is the "pop" of the variable stack.
?$	Evaluates the top of stack token as a variable name and pushes on the number of values that have been assigned to it. This is the "stack depth" of the variable.
->	Evaluates the top of stack token as a variable that resolves to a value that is evaluated as an RPNLang expression. Literally equivallent to ? ()