Re: [TUHS] Dennis Ritchie's couch

Bakul Shah <bakul(a)iitbombay.org> wrote: Yes, it's a fairly straightforward to write an operator precedence parser using a pushdown automaton. Knuth wrote about it in 1962 :-) [page 8] https://archive.org/details/bitsavers_computersA_13990695 I crufted together a #if evaluator for unifdef nearly 20 years ago, but it's, um, rather messy. (I was hacking on Dave Yost's code from the 1980s) http://dotat.at/cgi/git/unifdef.git/blob/HEAD:/unifdef.c#l950 More recently I learned how to write a Pratt parser (1973), which is just a particular style of operator precedence parser, but when it's done properly it can be really neat. https://dl.acm.org/doi/10.1145/512927.512931 The driver loop is tiny, if you set up the table mapping tokens to actions in the right way, so that the state transition functions handle errors as well as happy-path evaluation. (Blame Pratt for the field names!) static Value eval(Parser *p, int rbp) { Value v = token[p->tok].nud(p); while(token[p->tok].lbp > rbp) v = token[p->tok].led(p, v); return(v); } Vaughan Pratt also designed the pretty neat Sun logo. Tony. -- f.anthony.n.finch <dot(a)dotat.at> https://dotat.at/ Channel Islands: Variable or northeasterly 1 to 3, becoming west to southwest 2 to 4 late evening, backing south to southeast soon after dawn, veering southwest to west 3 to 5 by noon, locally variable 1 to 3 in the far south of the area. Smooth or slight, with a low swell developing later. Mist and fog patches clearing early by early afternoon showers at first and again later rain and drizzle for a time, overnight and in the morning. Moderate to good, occasionally poor locally very poor.