Kernel Syntax¶
The rules from context-free and lexical syntax are translated into kernel syntax by the SDF3 normalizer. When writing kernel syntax, one has more control over the layout between symbols of a production.
As part of normalization, among other things, SDF3 renames each symbol in the lexical syntax to include the suffix -LEX and each symbol in the context-free syntax to include the suffix -CF.
For example, the two productions
lexical syntax
BinaryConst = [0-1]+
context-free syntax
Block.Block = "{" Statement* "}"
written in kernel syntax look like
syntax
Block-CF.Block = "{" LAYOUT?-CF Statement*-CF LAYOUT?-CF "}"
BinaryConst-LEX = [0-1]+
Literals and character classes are lexical by definition, thus they do not need any suffix.
Note that each symbol in kernel syntax is uniquely identified by its full name including -CF and -LEX.
That is, two symbols named Block-CF and Block are different, if both occur in kernel syntax.
However, Block-CF is the same symbol as Block if the latter appears in a context-free syntax section.
As mentioned before, layout can only occur in between symbols if explicitly specified.
For example, the production
syntax
Block-CF.Block = "{" Statement*-CF LAYOUT?-CF "}"
does not allow layout to occur in between the opening bracket and the list of statements.
This means that a fragment such as:
{
x = 1;
}
would not be recognized as a block.
Created: April 19, 2024