Parsing negation

I found a chapter called “Object Pascal Grammar” in the Delphi 6 Object Pascal Language Guide, and have been using that as a starting point for my Delphi parser. It gives a decent (though incomplete) overview of the parser, but nothing on what the lexer should look like.

(For those curious to see it for yourselves, download d62pro.zip, open del6op.hlp, and search the index for “grammar”. Or you can get the Delphi 5 Object Pascal Language Guide as a zipped PDF.)

Here’s something I found really peculiar:

SimpleExpression -> ['+' | '-'] Term [AddOp Term]...
Term -> Factor [MulOp Factor]...

The peculiar thing is that the ‘+’ and ‘-‘ prefixes are introduced in SimpleExpression. This is weird on several levels. First of all, when I took math, we learned that the negation operator binds more tightly than multiplication. “-A” should parse as a Factor, not two levels above that as a SimpleExpression.

On a more practical level, this grammar would disallow some language constructs that I would expect to be perfectly reasonable, such as:

A + -B

Now, I suppose you could just write A – B instead. But this one would also be disallowed:

A * -B

Odd. I don’t have Delphi installed on my home PC, but I’ll have to check tomorrow and see whether these actually are accepted. If so, the grammar documentation is hosed.

(Just to test, I downloaded and installed the FreePascal compiler, and it happily parses both of the above. But according to a Pascal EBNF grammar I found, neither one should parse, and neither should A * -1, because numbers are inherently unsigned — if the grammar is right (and it does carry a big disclaimer), numbers only obtain signs once they become SimpleExpressions. Very strange.)

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