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 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 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. A synonym for the nul keyword that might be used in its place in such a situation is clearcontext. 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.

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