nul

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Summary

The nul statement has two purposes: in the output of a rule, it signifies deleting context and keystroke, and at the start of the context it signifies that the context buffer in the application must be empty (or no longer than the context of the rule) in order for the rule to match.

Syntax

nul [...] [+ key] > output
[...] [+ key] > nul

Description

nul in output

The nul statement will delete the context and key on the left hand side of the rule from the output; it is equivalent to having an empty output (which is not allowed).

If the nul statement is used on the right hand side of the rule, text or text-emitting statements cannot be used with it. The following statements can be used together with nul in the output:

• beep()
• call()
• reset()
• return
• save()
• set()
• use()

nul in context

nul can be used in the context in one special situation. It is used at the start of the context to tell Keyman to find a match only if the nul statement is before the start of the context buffer. For example, you may use this to test if the user is typing a character at the start of a paragraph, and use a different character in this situation.

Be careful when using nul in the context. In legacy environments, such as many Windows applications, the context buffer will be empty after cursor movement, menu access, or many other situations. This means that the nul keyword may match in situations where you are not expecting it, for example in the middle of a word.

In modern environments, such as touch devices, web keyboards, and some modern Linux, macOS, and Windows applications, the nul keyword will match at the start of a text buffer, and nowhere else. For example, in Microsoft Word, it will match at the start of the document, after a page, column or section break, at the start of a header or footer, at the start of a table cell and other similar situations.

Note

If the context in a rule has both an if() statement and a nul statement, then the nul statement must come first.

Examples

Example: Using nul on the right hand side of the context

This example will delete a visible Malayalam virama when a is typed.

U+0D4D + 'a' > nul

Example: Using nul on the left hand side of the context

This example will capitalize a letter at the start of a buffer.

nul + 'a' > 'A'

[!Hint] This is not the recommended approach to casing. See Casing Support for better ways to implement this.

Platforms

The nul statement can be used in keyboards on all platforms.

Windows	macOS	Linux	Desktop web	Mobile web	iOS	Android
✔	✔	✔	✔	✔	✔	✔

Version history

• Keyman 18: Clarification of acceptable uses of nul and other statements in output.
• The nul statement was introduced in Keyman 3.0.

See also

• context statement