Nullability

Grammar tests > Nullability

The warning

"X" is nullable

appears, if the production "X" matches an empty text. For example "X" may be defined by:

X ::= "la" *

This rule will recognize a text like "la la la", but also the empty text "" matches, because the '*"-repeat matches null repeats. In contrast, the production

X ::= "la" + 

is not nullable.

This warning can be a hint, that a nullable alternative was created inadvertently.

Productions, which only consist of semantic actions, in principle, are nullable too. However a warning will not be created in this case, because such a rule cannot lead to a wrong recognition, but can be uses as helping function.

The creation of this warning can be suppressed by an according checkbox in the project options.

This page belongs to the TextTransformer Documentation