Syntax

Basics

Statements

A gpss.py program contains a bunch of Statements that create a model. There is one Statement per line. For example:

GENERATE
QUEUE
ADVANCE
DEPART
TERMINATE

But this is no good — the Statements need Operands!

Operands

Most Statements will take Operands. These Operands appear directly after the Statement on the same line, separated from the Statement by whitespace. Individual Operands are separated with commas, with no whitespace in between.

Here’s an example of a really short Transaction program:

GENERATE    18,6
TERMINATE   1

In the example above, there are two Statements: GENERATE and TERMINATE. Both of these Statements have Operands: the GENERATE Block has an A Operand of 18 and a B Operand of 6. The GENERATE Block can take more Operands, but they’re optional, so if left out, default values are assumed. The TERMINATE Block has just one Operand specified, the A Operand, and its value is 1.

To skip over Operands (e.g. specifying A and C but not B), simply leave its spot blank, like so:

GENERATE    10,,50

The above specifies 10 as the A Operand and 50 as the C Operand, but it doesn’t specify the B Operand, making the B Operand assume its default value.

Comments

Comments in a gpss.py program are started with either a semicolon (;) or an asterisk (*). These can be placed anywhere, and everything that follows a comment character will be ignored by gpss.py.

; I'm a comment!
* I'm a comment too!

GENERATE    1   ; That GENERATE is a Statement, but I'm still a comment!
TERMINATE       * I'm also still a comment!

Labels

Labels can be declared in two ways:

• Before the Statement it is “tagging”, on the same line, optionally followed by a colon:

  Wait    ADVANCE     12      ; This ADVANCE Block has the label "Wait"
  Cook:   SEIZE       Oven    ; This SEIZE Block has the label "Cook"
  

  (The option not to use a colon is provided for compatibility with the original GPSS.)

• On its own line, with the label name followed by a colon:

  ExtremelyCoolLabel:
          QUEUE       Line    ; This QUEUE Block has the label "ExtremelyCoolLabel"
  

Notes

• Blank lines are allowed and ignored.
• gpss.py only cares about the case of entity names. For anything else, it couldn’t care less.
  • GENERATE and gEnErAtE are equivalent.
  • QUEUE Line and QUEUE line would join 2 different Queues.
  • Case of keyword Operands is also ignored.
• Amount of whitespace between fields doesn’t matter, as long as there’s some.
  • OK: GENERATE 1
  • Not OK: GENERATE1
• There are no restrictions on entity names other than that they must be non-empty (or not all whitespace), since that would likely be a mistake.

Differences from original GPSS

gpss.py programs look very similar to what an original GPSS program would look like, but there are a few differences:

• Fields don’t have to be placed at specific columns, but need to be separated by whitespace.
• Comments must start with a semicolon (;) or an asterisk (*), but can appear anywhere.
• A SIMULATE Block does not have to be present in the program to run the simulation. To only check for syntax errors, run gpss.py with the --no-sim or -S flag.