4 ms·

I think if you actually try this, you will find out why it's not a feasible concept (or why it's so vague as not to be useful). I think Hjelsberg and the C# te

by chubot 9y ago

I think if you actually try this, you will find out why it's not a feasible concept (or why it's so vague as not to be useful). I think Hjelsberg and the C# team know all about grammars. In particular, I know that using a BNF is not useful for shell, having ported the POSIX grammar to ANTLR. http://www.oilshell.org/blog/tags.html?tag=parsing#parsing http://www.oilshell.org/blog/tags.html?tag=parsing#parsing

WorldMaker 9y ago
Thinking that you can describe every programming language in BNF is similar to the RegEx problem: just as not every language is a Regular language that can be easily described as a single RegEx, not every programming language is (solely) a Context-Free Language with a grammar that can adequately be described in just BNF (or any other CFG description language) without ambiguity. (Some programming languages are context-sensitive, some programming languages allow ambiguity in the main grammar and have precedence rules or "tie-breakers" to deal with those situations.) "Universal grammar engines" and "Universal ASTs" are wondrous dreams had by many academics and like the old using RegEx in the wrong place adage: now you have N * M more problems to solve.
- chubot 9y ago
  Right, CFGs are insufficient for most languages, and yacc is also insufficient for most languages. Yacc is simultaneously less powerful and more powerful than CFGs. Less powerful because it uses the LALR(1) subset of CFGs, and more powerful because you can embed arbitrary code with semantic actions. I think the OP is looking for something more like this. This paper is from the same author as Nix, so he has some credibility. But I think this paper is not well written, and I'm not sure about the underlying ideas either. (The output of these pure declarative parsers is more complicated to consume as far as I remember.) Still, the paper does shows how much more there is to consider than "BNF". Thinking "BNF" will solve the problem is a naive view of languages, and as you point out, is very similar to the problem with not understanding what languages "regexes" can express. http://eelcovisser.org/post/135/pure-and-declarative-syntax-definition%3A-paradise-lost-and-regained http://eelcovisser.org/post/135/pure-and-declarative-syntax-... Mainstream parser generators pose restrictions on syntax definitions that follow from their implementation algorithm. They hamper evolution, maintainability, and compositionality of syntax definitions.

by chubot 9y ago

WorldMaker 9y ago

Thinking that you can describe every programming language in BNF is similar to the RegEx problem: just as not every language is a Regular language that can be easily described as a single RegEx, not every programming language is (solely) a Context-Free Language with a grammar that can adequately be described in just BNF (or any other CFG description language) without ambiguity. (Some programming languages are context-sensitive, some programming languages allow ambiguity in the main grammar and have precedence rules or "tie-breakers" to deal with those situations.) "Universal grammar engines" and "Universal ASTs" are wondrous dreams had by many academics and like the old using RegEx in the wrong place adage: now you have N * M more problems to solve.

chubot 9y ago
Right, CFGs are insufficient for most languages, and yacc is also insufficient for most languages. Yacc is simultaneously less powerful and more powerful than CFGs. Less powerful because it uses the LALR(1) subset of CFGs, and more powerful because you can embed arbitrary code with semantic actions. I think the OP is looking for something more like this. This paper is from the same author as Nix, so he has some credibility. But I think this paper is not well written, and I'm not sure about the underlying ideas either. (The output of these pure declarative parsers is more complicated to consume as far as I remember.) Still, the paper does shows how much more there is to consider than "BNF". Thinking "BNF" will solve the problem is a naive view of languages, and as you point out, is very similar to the problem with not understanding what languages "regexes" can express. http://eelcovisser.org/post/135/pure-and-declarative-syntax-definition%3A-paradise-lost-and-regained http://eelcovisser.org/post/135/pure-and-declarative-syntax-... Mainstream parser generators pose restrictions on syntax definitions that follow from their implementation algorithm. They hamper evolution, maintainability, and compositionality of syntax definitions.