Understanding and Replacing Deprecated Jam5 Syntax
with Jam7 Syntax

JPL Variable and Field Assignment -- Using Cat to Assign Values

The cat verb was used to make variable assignments in Jam5. The target (or l-value) of the assignment was the first word after the cat verb. Targets could be either a variables defined in jpl with vars, fields on the screen or references to fields on the screen. The rest of the statement following the target was the source (or r-value) of the assignment and could be a combination of variables, field names, references to fields or hard-coded constants. Colon expansion occured before the variable assignment occured and both the target and the source could contain variables to be colon-expanded.

In Jam5, the cat verb was used for variable and widget assignment and there were two different targets of cat statements

  1. variable/widget names
  2. field references to widgets on the current screen

If the target of a cat assignment begins with a pound sign (#), a plus sign (+) or a minus sign (-), then it is a field reference-this is easily seen because variable names cannot have these characters as part of the name. Regular variable names consist of alphanumeric characters, the underscore (_), the exclamation point (!), the period (.) and the dollar sign ($). (Note that the ! was added to variable names in Jam7-the rest of these characters could have been used in variable names in Jam5)