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XS::APItest::KeywordRPN - write arithmetic expressions in RPN


use XS::APItest::KeywordRPN qw(rpn calcrpn);

$triangle = rpn($n $n 1 + * 2 /);

calcrpn $triangle { $n $n 1 + * 2 / }


This module supplies plugged-in keywords, using the new mechanism in Perl 5.11.2, that allow arithmetic to be expressed in reverse Polish notation, in an otherwise Perl program. This module has serious limitations and is not intended for real use: its purpose is only to test the keyword plugin mechanism. For that purpose it is part of the Perl core source distribution, and is not meant to be installed.

RPN expression syntax

Tokens of an RPN expression may be separated by whitespace, but such separation is usually not required. It is required only where unseparated tokens would look like a longer token. For example, 12 34 + can be written as 12 34+, but not as 1234 +.

An RPN expression may be any of:


A sequence of digits is an unsigned decimal literal number.


An alphanumeric name preceded by dollar sign refers to a Perl scalar variable. Only variables declared with my or state are supported. If the variable's value is not a native integer, it will be converted to an integer, by Perl's usual mechanisms, at the time it is evaluated.

A B +

Sum of A and B.

A B -

Difference of A and B, the result of subtracting B from A.

A B *

Product of A and B.

A B /

Quotient when A is divided by B, rounded towards zero. Division by zero generates an exception.

A B %

Remainder when A is divided by B with the quotient rounded towards zero. Division by zero generates an exception.

Because the arithmetic operators all have fixed arity and are postfixed, there is no need for operator precedence, nor for a grouping operator to override precedence. This is half of the point of RPN.

An RPN expression can also be interpreted in another way, as a sequence of operations on a stack, one operation per token. A literal or variable token pushes a value onto the stack. A binary operator pulls two items off the stack, performs a calculation with them, and pushes the result back onto the stack. The stack starts out empty, and at the end of the expression there must be exactly one value left on the stack.


These are the operators being added to the Perl language.


This construct is a Perl expression. EXPRESSION must be an RPN arithmetic expression, as described above. The RPN expression is evaluated, and its value is returned as the value of the Perl expression.


This construct is a complete Perl statement. (No semicolon should follow the closing brace.) VARIABLE must be a Perl scalar my variable, and EXPRESSION must be an RPN arithmetic expression as described above. The RPN expression is evaluated, and its value is assigned to the variable.


This module only performs arithmetic on native integers, and only a small subset of the arithmetic operations that Perl offers. This is due to it being intended only for demonstration and test purposes.

The RPN parser is liable to leak memory when a parse error occurs. It doesn't leak on success, however.


Devel::Declare, "PL_keyword_plugin" in perlapi


Andrew Main (Zefram) <>


Copyright (C) 2009 Andrew Main (Zefram) <>


This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.