context free grammars: Difference between revisions
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aka CFG, a grammar which rules do not depend on the context during parsing. I know, this is a circular definition. | |||
Here are some examples : | |||
* Lojban 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: | |||
{{mu||you if sense makes sentence this, please read the first six words backwards.}} | |||
==Discussion== | |||
*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, similar to \ line continuations in C. | |||
*** they are trivially handled by the lexer. | |||
*But this sentence isn't legitimate English... | |||
**.djorden.: | |||
* | ***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. | ||
*[[User:And Rosta|And Rosta]]: | |||
**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. | |||
*[[Jay Kominek|Jay]]: | |||
**Fairly sure that is the case. It would certainly be context. | |||
*.djorden.: | |||
**The above is correct - a context free grammar may only have one non terminal on the left hand side of its rules. | |||
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Latest revision as of 14:40, 1 July 2015
aka CFG, a grammar which rules do not depend on the context during parsing. I know, this is a circular definition.
Here are some examples :
- Lojban 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. |
Discussion
- 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, similar to \ line continuations in C.
- they are trivially handled by the lexer.
- 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?
- But this sentence isn't legitimate English...
- .djorden.:
- 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.
- .djorden.:
- And Rosta:
- 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.
- Jay:
- Fairly sure that is the case. It would certainly be context.
- .djorden.:
- The above is correct - a context free grammar may only have one non terminal on the left hand side of its rules.