Owl: Parser generator for visibly pushdown languages • Hacker News Board

Re: Owl: Parser generator for visibly pushdown languages

#31

Post by nicoburns » Thu, Nov 08, 2018, 1:38 AM UTC

jhpriestley wrote: post #22

chrisseaton wrote: post #18

Earlier quoted context omitted.

> You can report the location where the parse failed, what was recognized before that point, what tokens would have been allowed to follow. This is a garbage 'computer says no' kind of error message. It doesn't tell you why those tokens would have been allowed to follow, why it is that those are the only tokens allowed to follow, why the token you used is not one of the allowed one, or what you could have more likely…

What context or data would constitute a helpful error message in your opinion? Do you have an example of a recursive-descent parser that generates particularly good error messages?

See elm and rust for examples of compilers with good error messages.

Re: Owl: Parser generator for visibly pushdown languages

#32

Post by grandmczeb » Thu, Nov 08, 2018, 2:16 AM UTC

nicoburns wrote: post #31

jhpriestley wrote: post #22

Earlier quoted context omitted.

What context or data would constitute a helpful error message in your opinion? Do you have an example of a recursive-descent parser that generates particularly good error messages?

See elm and rust for examples of compilers with good error messages.

Or clang.

Re: Owl: Parser generator for visibly pushdown languages

#33

Post by theknarf » Thu, Nov 08, 2018, 11:11 AM UTC

ccmcarey wrote: post #2

I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…

Just write down the grammar by hand first, then write the recursive descent parser strictly based on the grammar.

Re: Owl: Parser generator for visibly pushdown languages

#34

Post by ltratt » Thu, Nov 08, 2018, 11:51 AM UTC

dbcurtis wrote: post #13

ccmcarey wrote: post #2

I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…

The problem with yacc/bison-style LALR parser generators is that you end up with an LALR parser. Which works great on syntactically correct code, but trying to get a reasonable error message out of one is about as much fun as repeatedly poking yourself in the eye with a sharp stick. Also, by the time you add the empty productions that you want to help with code generation, the grammar gets delicate and brittle. OTOH,…

> The problem with yacc/bison-style LALR parser generators is that you end up with an LALR parser. Which works great on syntactically correct code, but trying to get a reasonable error message out of one is about as much fun as repeatedly poking yourself in the eye with a sharp stick.

The standard ways of getting error messages from LR parsers are pretty bad, but there is a long line of work that's tried to make it better. If you'll forgive the immodesty on my part, I've been working on updating and fixing that work. There's a draft paper at https://arxiv.org/abs/1804.07133 and a beta-ish implementation at https://github.com/softdevteam/grmtools/, so you can see what you think!

[EDIT: fixed formatting]