Using Groups

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Groups provide a facility for grouping related rules, similar to functions in other programming languages. When used effectively, groups can reduce the size and complexity of your keyboards significantly.

Groups can match on context only, or on context and keystroke. The context-only groups can be very useful for pre- and post-processing rules, such as reordering stacked diacritics.

In Keyman 15.0 and later versions, groups can also be read-only. This is important for contexts where emitting output is not possible, such as when a new context is selected.

Only the first rule in the group that matches will be fired. There are two special rules, match and nomatch that will then fire if a rule is either matched or not matched, respectively. The criteria for not matching is a little more nuanced, however, as a keystroke is deemed to have not matched only if it would have generated a character anyway, so F1 would not fire the nomatch rule, unlike a character key.

Rules in a group have a special order. They are ordered first by length of context, with longest context first, and then by line order in the file. This slightly non-intuitive ordering makes it much simpler to group rules according to their function, rather than necessarily by their priority.

The examples below show the three types of groups.

group(mygroup)                   c context only
  'a' > 'b'

group(mygroup) readonly          c read only
  'a' > context set(aPressed = '1')

group(mygroup) using keys        c context and keystroke
  'a' + 'a' > 'c'

using keys clause

To tell Keyman that the group should include key processing, you should include the using keys section of the statement; if that is left out, the group checks the context only. The keystroke will remain the same during processing; you can have many groups that each use using keys, and the keystroke will be the same for all of them. The key section of a rule (including the + sign) is not valid for context processing groups.

readonly clause

Read-only groups are new in Keyman 15. They are used primarily from NewContext and PostKeystroke entry points, and indicate that text output is not permitted from rules within the group. The purpose of these groups is typically to change the current layer of the touch keyboard, or perform similar state updates.

use() statements in a read-only group may only reference other read-only groups.

An implicit context statement is added to the front of every rule output in a read-only group, if it is not already present, to ensure that the input context is not modified. As it is not legal to have an empty output, you may use the context statement, not the nul statement on the right-hand side of an otherwise empty output.

In a read-only group, only the following output statements are permitted:

• call() - the custom function must not attempt to do any output
• context - only at the start of the output
• use() - use other readonly groups
• set()
• reset()
• save()

See Casing Support for a comprehensive example of how read-only groups are used.

use statement

The use() statement allows you move into another group when a rule is matched. All output from the current group preceding the use() statement is processed into the context before the subsequent group is entered (although it is not sent to the application until processing finishes for the current keystroke). For example:

c This example prohibits two vowels in a row
store(vowel) 'aeiouAEIOU'

begin Unicode > use(precheck)

group(precheck)
any(vowel) > context use(precedingVowel)
nomatch > use(main)    c Preceding character is not a vowel. Do normal processing.

group(precedingVowel) using keys
+ any(vowel) > beep
nomatch > use(main)    c This key is not a vowel. Do normal processing.

group(main) using keys
c Main processing goes here

Some important things to note from this example:

The first rule (any(vowel) > context use(precedingVowel)) uses the context statement to copy the matched context to the output, so that Keyman can move it back into the context for use with the precedingVowel group. If you do not do this, the context will be dropped before precedingVowel is used, and the character will be deleted from the screen.

Note: the match rule will not be fired if the rule in the group that was matched includes a use() statement in its output. A workaround for this situation is to duplicate the match rule output in the output of rules that contain use() statements. A more comprehensive solution may be included in future versions of the language.

Empty final group

If the final group processed is a context and keystroke group (using keys), and there is no nomatch rule, and the keystroke is not matched in the group, the keystroke will be output to the screen, regardless of whether or not it was matched in earlier groups.

This can be used for example if you want to process a change at the end of a word which might be the last word in a line or in a text field, where the user presses Enter or Tab:

group(main) using keys

+ 's' > 'σ'
'σ + [K_ENTER] > 'ς' use(final)

group(final) using keys
c Empty final group causes keystroke to be emitted

Statements and rules used with groups

The following statements and special rules are used with groups:

begin rule : Defines the starting group for the keyboard layout

group() statement : Starts a new group of rules

match rule : A system rule that is fired when another rule in the group is matched

nomatch rule : A system rule that is fired when no rule is matched

return statement : Stops processing of the current keystroke

use() statement : Starts processing in another group