5 ms·

Precedence is actually not hard. The way I do it is: (1) Parse everything as though it were left-to-right. (2) After each node is parsed, look at its immediat

by jblow 10y ago

Precedence is actually not hard. The way I do it is: (1) Parse everything as though it were left-to-right. (2) After each node is parsed, look at its immediate descendants and rearrange links as necessary. (Nodes in parentheses are flagged so you don't rearrange them.) I can tell that the person above who is listing off a bunch of reasons not to use "recursive descent" hasn't written a compiler by hand ever (or not well). Most of the things he is talking about are easier to do by hand than in some complicated and relatively inflexible system. Note that 'prediction' is mostly a red herring since you can look as many tokens ahead as you want before calling the appropriate function to handle the input. You would need to have a pathologically ambiguous language in order to make that part hard, and if your language is that ambiguous, it is going to confuse programmers! In general, parsing is easy (if you know how to program well in the first place) and is only made more difficult/inflexible/user-unfriendly by using parsing tools. That doesn't mean that academic theories about parsing are bad -- it's good that we understand deeply things about grammars -- but that does not mean you should use those systems to generate your source code. (I do think it's a good idea to use a system like that to spot ambiguities in your grammar and decide how to handle hem, because otherwise it's easy to be ignorant... But I would not use them to generate code!)

chubot 10y ago
Precedence isn't hard -- but it's often not done with recursive descent. In other words, it's not just "typing out the code". I think you're misreading my reply. A hand-written recursive descent works a lot of the time, and is almost certainly the right "default". But my point is that it doesn't work all the time. If it works for you, great! That doesn't mean all parsing is easy. Try adding an autoformatter like gofmt or IDE completion for Jai, and see how your parser changes (or if you have to write an entirely new parser). In particular, I'm NOT advocating that all parsers should written with parsing tools. I think you must have read that somewhere else. In fact, I used EXACTLY the same strategy as you mention on my recent shell parser. I machine-checked the grammar with ANTLR, and wrote some test cases for it. Then I transcribed that grammar to recursive descent by hand, along with several other ad hoc strategies that you need to parse an ad hoc language like shell. (ANTLR is a great front end, but not necessarily a good back end. The generated Java code is probably OK, but the C++ code it generates is preposterous. Yacc is not that much better, i.e. with globals all over the place.) "Just type out the code" is a horrible strategy because you will end up not knowing what language you've designed. If your post advocated "prototype your grammar with a tool, and then manually transcribe the grammar to code", then I'd be inclined to agree. But that's not what you said. It seems like you are more interested in telling everyone what a great programmer you are, and how things are so easy for you, rather than spreading any interesting knowledge about parsing.
- jblow 10y ago
  "Try adding an autoformatter like gofmt or IDE completion for Jai, and see how your parser changes (or if you have to write an entirely new parser)." It would not change at all, and I have no idea why you think it would, except to guess that the model you have in your head of a hand-written parser kind of sucks. They don't have to suck. "...not knowing what language you've designed." I have no idea what you're on about here either. Look, I think you are making things a lot harder than they are. I am not bragging ... I used to build lexers and parsers by hand 23+ years ago when I was a student in college and had almost no programming experience compared to what I have now. It is not hard. If you think it's hard, something is missing in your knowledge set. (I also built stuff using parser tools 23+ years ago, and was able to very clearly contrast the two methods. Parser tools have gotten slightly better since then, but not much.)
  - chubot 10y ago
    I don't think you've really thought about those things, or you don't understand what I'm saying. Here is the recent video about Heljsberg I mentioned: https://news.ycombinator.com/item?id=11685317 https://news.ycombinator.com/item?id=11685317 I doubt your parser is even close to doing what he is talking about. I think you're an advocate of Microsoft's debugger over GDB, so hopefully what they are saying carries some weight (i.e. a little computer science doesn't hurt). They aren't just a bunch of eggheads; they are actually building language tools that people use. And again, I'm not claiming that ALL parsing is hard. Sometimes you can just write a hand written lexer and recursive descent parser and be done with it. But that's not true in all cases, particularly for "real world" languages, or when you are designing a brand new language. Saying parsing is easy is like saying "I wrote a game in BASIC in 8th grade, so writing games is easy". (And yes I've actually heard that statements like that ... ) I was researching Jai awhile back, and was definitely curious about it. I don't think you have released the code, so I can't tell which of these statements is true: 1) Parsing is easy for you 2) You think parsing is easy, but your language and your parser are full of bugs I'm perfectly willing to believe #1 (honestly). But that doesn't mean all parsing is easy, for say a competent programmer in some other domain. It doesn't help anyone to say "just type out the code". Your comment about how you do precedence is a little more helpful.

by jblow 10y ago

chubot 10y ago

Precedence isn't hard -- but it's often not done with recursive descent. In other words, it's not just "typing out the code". I think you're misreading my reply. A hand-written recursive descent works a lot of the time, and is almost certainly the right "default". But my point is that it doesn't work all the time. If it works for you, great! That doesn't mean all parsing is easy. Try adding an autoformatter like gofmt or IDE completion for Jai, and see how your parser changes (or if you have to write an entirely new parser). In particular, I'm NOT advocating that all parsers should written with parsing tools. I think you must have read that somewhere else. In fact, I used EXACTLY the same strategy as you mention on my recent shell parser. I machine-checked the grammar with ANTLR, and wrote some test cases for it. Then I transcribed that grammar to recursive descent by hand, along with several other ad hoc strategies that you need to parse an ad hoc language like shell. (ANTLR is a great front end, but not necessarily a good back end. The generated Java code is probably OK, but the C++ code it generates is preposterous. Yacc is not that much better, i.e. with globals all over the place.) "Just type out the code" is a horrible strategy because you will end up not knowing what language you've designed. If your post advocated "prototype your grammar with a tool, and then manually transcribe the grammar to code", then I'd be inclined to agree. But that's not what you said. It seems like you are more interested in telling everyone what a great programmer you are, and how things are so easy for you, rather than spreading any interesting knowledge about parsing.

jblow 10y ago
"Try adding an autoformatter like gofmt or IDE completion for Jai, and see how your parser changes (or if you have to write an entirely new parser)." It would not change at all, and I have no idea why you think it would, except to guess that the model you have in your head of a hand-written parser kind of sucks. They don't have to suck. "...not knowing what language you've designed." I have no idea what you're on about here either. Look, I think you are making things a lot harder than they are. I am not bragging ... I used to build lexers and parsers by hand 23+ years ago when I was a student in college and had almost no programming experience compared to what I have now. It is not hard. If you think it's hard, something is missing in your knowledge set. (I also built stuff using parser tools 23+ years ago, and was able to very clearly contrast the two methods. Parser tools have gotten slightly better since then, but not much.)
- chubot 10y ago
  I don't think you've really thought about those things, or you don't understand what I'm saying. Here is the recent video about Heljsberg I mentioned: https://news.ycombinator.com/item?id=11685317 https://news.ycombinator.com/item?id=11685317 I doubt your parser is even close to doing what he is talking about. I think you're an advocate of Microsoft's debugger over GDB, so hopefully what they are saying carries some weight (i.e. a little computer science doesn't hurt). They aren't just a bunch of eggheads; they are actually building language tools that people use. And again, I'm not claiming that ALL parsing is hard. Sometimes you can just write a hand written lexer and recursive descent parser and be done with it. But that's not true in all cases, particularly for "real world" languages, or when you are designing a brand new language. Saying parsing is easy is like saying "I wrote a game in BASIC in 8th grade, so writing games is easy". (And yes I've actually heard that statements like that ... ) I was researching Jai awhile back, and was definitely curious about it. I don't think you have released the code, so I can't tell which of these statements is true: 1) Parsing is easy for you 2) You think parsing is easy, but your language and your parser are full of bugs I'm perfectly willing to believe #1 (honestly). But that doesn't mean all parsing is easy, for say a competent programmer in some other domain. It doesn't help anyone to say "just type out the code". Your comment about how you do precedence is a little more helpful.