bzs@bu-cs.UUCP (Barry Shein) (11/21/85)
And the answer is...ya can't do it. At least not in any useful way. Readmacros are one good counter-example, hunks and comma syntax ((a . b . c) and (a , b , c)) were all forgotten for a while. Super parens (closing off some set of open parens, either all or up to the last super-open such as "<defun fact (a) (cond ((< a 2) 1)(t ....>" or some such are hard I guess. The real answer is, take it from people who have built a (read), it's hairy and BNF doesn't help (now consider things like backquote syntax.) It's a nice fantasy but not useful. -Barry Shein, Boston University
israel@umcp-cs.UUCP (Bruce Israel) (11/21/85)
In article <771@bu-cs.UUCP> bzs@bu-cs.UUCP (Barry Shein) writes: > Super >parens (closing off some set of open parens, either all or up to >the last super-open such as "<defun fact (a) (cond ((< a 2) 1)(t ....>" >or some such are hard I guess. At our site we had this discussion about a lisp BNF a few years ago. At the time I came up with the following non-ambiguous BNF that handles '.'s as well as the '[' & ']' super-parens. S ::= SEXPR S ; lisp is a sequence of s-exprs | <empty> ; (possibly empty) SEXPR-list ::= S ; a list of sexprs ... | S SEXPR . SEXPR ; maybe dotted SEXPR ::= '(' SEXPR-list ')' ; sexpr has matching )'s, | '[' SEXPR-list ']' ; or matching ]'s, | <atom> ; or none, | '(' S-MINUS-] ')' ; or matching that close off | '[' S-MINUS-) ']' ; one inside, ! S-MINUS-)-1 ']' ; or non-matching that is ! S-MINUS-]-1 ')' ; closed by an unmatched one S-MINUS-] ::= SEXPR-list S-MINUS-]-1 ; can be closed by a ')' ; because its a list of sexprs ; where the last one ends ; without a ']' S-MINUS-) ::= SEXPR-list S-MINUS-)-1 ; ditto S-MINUS-]-1 ::= '[' SEXPR-list ; either the ')' closes one, | '[' S-MINUS-] ; or more than one. S-MINUS-)-1 ::= '(' SEXPR-list ; ditto | '(' S-MINUS-) -- Bruce Israel University of Maryland, Computer Science Dept. {rlgvax,seismo}!umcp-cs!israel (Usenet) israel@Maryland (Arpanet)
barmar@mit-eddie.UUCP (Barry Margolin) (11/24/85)
In my previous posting about the reason that a parser must know about
read macros, I meant to also mention that a parser must also know about
the SETSYNTAX functions. Using this, a programmer can reprogram the
meaning of any character. Thus, I could tell the reader to treat any
character as equivalent to parentheses, the cons dot, quote, etc. The
BNF cannot take this into account.
--
Barry Margolin
ARPA: barmar@MIT-Multics
UUCP: ..!genrad!mit-eddie!barmar