Constraints are rules that restrict certain combinations from being typed.
Constraints can be made with any rules, even
Typical constraints would reject isolated keystrokes, keystrokes following a certain context, or certain context and keystroke combinations.
To make a keystroke produce no output, you use the
nul keyword. 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
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
beep statements is that the
nul statement is only valid as the whole output, while the
statement can be used as just a part of the output.
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
"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
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. A synonym for the
keyword that might be used in its place in such a situation is
As in both situations above, you can also use the
beep statement here to
alert the user with a warning beep:
"eras" + "e" > beep
For a typical keyboard, you might use a constraint in a
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.