Fraction interpolation

[Alex-Jordan]

I don't understand why the two strings printed here are different.

DOCUMENT();

loadMacros(qw(PGstandard.pl contextFraction.pl));
Context("Fraction");
$a = Fraction(1/2);
TEXT("$a x", $BR, "$a"." x");

ENDDOCUMENT();

The first makes (1/2) x and the second makes 1/2 x.

Note this is what I see in 2.19 and develop, so it's not something to do with the recent changes to Fraction. But I noticed this while debugging something. Is the string interpolation of $a within "$a x" somehow aware of what characters are adjacent to it?

[somiaj]

Looks like this is part of how MathObjects override perl operators and order of operations. Note I also see that $a . " x" becomes (1/2) x as well. There is a comment above the _dot method (the method that overrides .) in lib/Value.pm on lines 891-902 about this. It mentions that when MathObjects override the . operator, they put parenthesis around the MathObject to ensure the MathObject is treated as a single expression for possible future parsing.

The reason I think "$a" . " x" doesn't have parenthesis is because the first "$a" turns the MathObject $a into the string "1/2", and then when that string is concated it is no longer a MathObject, so it is just using the normal perl concatenation and not the overridden method for MathObjects.

[somiaj]

Note, one thing I discovered in testing your example problem (which didn't work because MathObjects.pl is not loaded) is that contentextFraction.pl doesn't load the MathObjects.pl macro. In this case it appears that we need to load MathObjects.pl in the contextExtensions.pl macro.

[dpvc]

I don't understand why the two strings printed here are different.

As @somiaj mentions, this has to do with the MathObject overrides. It turns out that there are two different ones involved, here: the concatenation operator (.) and the stringify operation (""). The first is called when . is used explicitly with a MathObject, or when a MathObject is inserted into a string that is not otherwise empty, while the second is called when it is inserted into a otherwise empty string. So your "$a x" is the former, while "$a" is the latter.

The concatenation operation is overridden by the MathObject _dot method, while stringify is override by the stringify method (both defined in pg/lib/Value.pm). The _pdot method is the one that ends up inserting parentheses, so that if you did something like "$a^2 x" when $a = Fraction(2/3), you would get "(2/3)^2 x" rather than the incorrect "2/3^2 x". The stringify method just calls the MathObject's string method, which does not insert parentheses.

The effect of "$a x" is to call $a->_dot(" x"), while "$a" is effectively $a->string. I wasn't actually aware of this distinction until I looked into the issue you raised.

If you think at "$a" should also produce "(1/2)", then the stringify method could be redefined to be

sub stringify {
  (shift)->_dot('');
}

in pg/lib/Value.pm. But if you are asking for "$a x" to produce "1/2 x", then I would recommend against that, as the parentheses are needed in cases like "$a^2 x", and there is no way for MathObjects to tell whether it is being inserted into a situation where the parentheses are needed or not.

[Alex-Jordan]

OK, thanks for the explanations. I have a follow-up question but now this has evolved to where I think it's better to post it to the forum. I'll do that later today.