Using constraint rules
Constraints are rules that restrict certain combinations from being typed. Constraints can be made with any rules, even match
or nomatch rules.
Typical constraints would reject isolated keystrokes, keystrokes following a certain context, or certain context and keystroke combinations.
Rejecting keystrokes
To make a keystroke produce no output, you use the nul statement. For example, we could use the
following rule to prevent the user from typing "Q":
+ "Q" > nul
In some cases, you might want to let the user know that they typed an illegal key; for this you would use the
beep statement. The beep statement, like the nul statement, produces
no output, but it plays the sound assigned to "Default Beep" in the Sounds Control Panel.
+ "Q" > beep
One difference between the nul and beep statements is that the nul statement is only valid
as the whole output, while the beep statement can be used as just a part of the output.
Rejecting keystrokes after a context
In order to reject a keystroke following a certain context, you make the rule output just the context. Logically, you use the
context statement for this:
"ba" + "d" > context
This would have the effect of ignoring the "d": the output would be the context, so to the user it would appear as though
they had never typed the "d". You can also use the beep statement here to aurally alert the user
that they typed a bad word.
"ba" + "d" > context beep
Rejecting both context and keystroke
In certain special situations, you might wish to reject both the context and the keystroke. You can use the nul
statement to achieve this aim:
"eras" + "e" > nul
This would effectively erase the context and the key. As in both situations above, you can also use the
beep statement here to alert the user with a warning beep:
"eras" + "e" > beep
Typical usage of constraints
For a typical keyboard, you might use a constraint in a nomatch rule, to prevent the user
from typing anything that is not matched by another rule.
Alternatively, you might want to restrict a few certain combinations: in this case you might put specific constraint rules in the
first group of a keyboard, and use a nomatch rule to process all other (allowed) input with another group. The separation
of legal and illegal input into separate groups will make the keyboard's operation clearer.
