Talk:Compiler/ Verifying Syntax

From Rosetta Code

Incorrect BNF[edit]

The BNF given does not permit = to be directly followed by not. Go/Julia/Perl all get that wrong (better to fix task tho?).
The Go output also claims that "g not = h" is valid, whereas Perl/Julia and now Phix call that invalid. --Pete Lomax (talk) 18:57, 3 January 2020 (UTC)

I may not understand the specification given for BNF syntax. Doesn't the rule:

expr_level_4 = ["not"] expr_level_5 [('=' | '<') expr_level_5] ;

mean any expr_level_5 expression can be preceded by not? --Wherrera (talk) 19:09, 3 January 2020 (UTC)

First/left one yes, second/right one no. Consider expr_level_4 = ["not"] expr_level_5 [('=' | '<') ["not"] expr_level_5] ;
Ideally the not would be moved into its own private rule, rather than clog up the (partial) comparison rule anyway. --Pete Lomax (talk) 19:31, 3 January 2020 (UTC)
Presumably the "not" applies to the relation as a whole, so "not a = b" means "not ( a = b )" and not "( not a ) = b". --Tigerofdarkness (talk) 22:01, 5 January 2020 (UTC)
Not sure, I would say that is undefined. Putting "not" in a separate rule would give it an unambiguous precedence. --Pete Lomax (talk) 23:44, 6 January 2020 (UTC)

Note the square brackets [] around the [('=' | '<') expr_level_5]. This means that the expression

'2 < 3 < 4'

should not be legal (i.e. a non-associative violation). [] means 0 or 1. {} means 0 or more. --Tybalt89 (talk) 22:28, 5 January 2020 (UTC)

Well spotted. Please sign your posts, with --~~~~ thanks --Pete Lomax (talk) 14:14, 4 January 2020 (UTC)
I didn't know how to sign :( --Tybalt89 (talk) 22:28, 5 January 2020 (UTC)

This problem is in dire need of a really extensive test suite. --Tybalt89 (talk) 22:28, 5 January 2020 (UTC)

That should be part of the task spec. --Pete Lomax (talk) 14:14, 4 January 2020 (UTC)