context free grammars: Difference between revisions

From Lojban
Jump to navigation Jump to search
m (Gleki moved page jbocre: context free grammars to context free grammars without leaving a redirect: Text replace - "jbocre: c" to "c")
m (Text replace - "jbocre: ([L-Z])" to "$1")
Line 4: Line 4:
Here are some examples :
Here are some examples :


* Lojban's grammar, and more generally any [[jbocre: LR|LR]] or [[jbocre: LL|LL]] grammar, are context-free.
* Lojban's grammar, and more generally any [[LR|LR]] or [[LL|LL]] grammar, are context-free.


* The C language is supposed to be context-free, except on the topic of type definition (if "typedef" preceeds a definition, the symbol defined becomes a type rather than a variable : semantics change depending on the context, given the same lexical grammar).
* The C language is supposed to be context-free, except on the topic of type definition (if "typedef" preceeds a definition, the symbol defined becomes a type rather than a variable : semantics change depending on the context, given the same lexical grammar).

Revision as of 14:47, 23 March 2014

A grammar which rules do not depend on the context during parsing. I know, this is a circular definition.

Here are some examples :

  • Lojban's grammar, and more generally any LR or LL grammar, are context-free.
  • The C language is supposed to be context-free, except on the topic of type definition (if "typedef" preceeds a definition, the symbol defined becomes a type rather than a variable : semantics change depending on the context, given the same lexical grammar).
    • C is fairly good about it, compared to many other languages.
  • English is not context-free. The way we parse/understand sentences depends on the context, either past or future. Consider this text :

you if sense makes sentence this, please read the first six words backwards.

  • mi'e cein. Doesn't Lojban have si/sa/su, which must be "understood" by the parser in the same sense as the English example above in order to be correctly parsed?
    • maybe this doesn't address what you mean -- but si/sa/su can be implemented below actual language parsing, simsa things such as \ line continuations in C.
    • they are trivially handled by the lexer.
  • But this sentence isn't legitimate English...
    • arguable; but the point still stands -- english isn't context free. A phrase structure grammar for english would be hideously large and type 0-1, if a complete one were ever made, which is unlikely. --mi'e .djorden.

I thought a CFG was one where the left side of the rewrite rule is unconditional. E.g. "A -> B C" is context free, but "A -> B C, when A is preceded by D" ("D A -> B C") is not context free. --And Rosta

Fairly sure that is the case. It would certainly be context. --Jay

the above is correct -- a context free grammar may only have one non terminal on the left hand side of its rules. --mi'e .djorden.