Talk:Compiler/Verifying syntax: Difference between revisions
→Incorrect BNF
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==Incorrect BNF==
The BNF given does not permit = to be directly followed by not. Go/Julia/Perl all get that wrong (better to fix task tho?).<br>
The Go output also claims that "g not = h" is valid, whereas Perl/Julia and now Phix call that invalid. --[[User:Petelomax|Pete Lomax]] ([[User talk:Petelomax|talk]]) 18:57, 3 January 2020 (UTC)
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: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. --[[User:Petelomax|Pete Lomax]] ([[User talk:Petelomax|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". --[[User:Tigerofdarkness|Tigerofdarkness]] ([[User talk: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. --[[User:Petelomax|Pete Lomax]] ([[User talk:Petelomax|talk]]) 23:44, 6 January 2020 (UTC)
Note the square brackets [] around the [('=' | '<') expr_level_5]. This means that the expression
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should not be legal (i.e. a non-associative violation).
[] means 0 or 1.
{} means 0 or more. --[[User:Tybalt89|Tybalt89]] ([[User talk:Tybalt89|talk]]) 22:28, 5 January 2020 (UTC)
:Well spotted. Please sign your posts, with <nowiki>--~~~~</nowiki> thanks --[[User:Petelomax|Pete Lomax]] ([[User talk:Petelomax|talk]]) 14:14, 4 January 2020 (UTC)
:: I didn't know how to sign :( --[[User:Tybalt89|Tybalt89]] ([[User talk:Tybalt89|talk]]) 22:28, 5 January 2020 (UTC)
This problem is in dire need of a really extensive test suite. --[[User:Tybalt89|Tybalt89]] ([[User talk:Tybalt89|talk]]) 22:28, 5 January 2020 (UTC)
:That should be part of the task spec. --[[User:Petelomax|Pete Lomax]] ([[User talk:Petelomax|talk]]) 14:14, 4 January 2020 (UTC)
Would it not be better if the EBNF included explicit placement of whitespace characters? As it stands, it seems to me one is left to one's own opinion as to where whitespace is allowed or required. (A separate lexical analysis stage would alleviate this difficulty, but we already have separated lex and parse tasks.) --[[User:Chemoelectric|Chemoelectric]] ([[User talk:Chemoelectric|talk]]) 18:40, 15 October 2022 (UTC)
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