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I'm attempting to do some hacking in some Git source code (as in the source code for Git, not just some random piece of code managed by Git). The bit I'm looking at is in Perl, and I'm having trouble understanding what's going on.

I have very little experience (and that several years old) of Perl; I've asked a couple of friends with more experience for advice, but they've turned up nothing.

The relevant bit of code is in the v1.8.1.5 source code, where git-svn.perl's cmd_fetch function includes the line:

$_fetch_all ? $gs->fetch_all : $gs->fetch;

My best reading of this is that it will call either the fetch or fetch_all functions (I can't see how it could be doing anything else, certainly).

In SVN.pm we find that fetch function, which starts with the following line:

my ($self, $min_rev, $max_rev, @parents) = @_;

I recognise that as collecting the function arguments, but (and finally, my question): where do these arguments get passed in?

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2 Answers 2

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The function called with the arrow notation is called as a method. The first argument to a method is the object whose method was called. $self will be therefore set to $gs. The rest of the arguments is empty, hence undef.

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Technically, the two scalar arguments get undef and the array argument gets the empty list.
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First, I really hope there is an assignment to the left of the code you cited. Using the conditional operator for control flow is a crime against humanity. That said, your intuition about what happens is correct: Depending on the value of $_fetch_all, either $gs->fetch or $gs->fetch_all is called. Now, on to the argument question.

Perl method calls pass arguments by prepending the invocant to the list of arguments, so the call

$gs->fetch

results in the arguments ($gs) being passed into the method as @_. The argument assignment line

my ($self, $min_rev, $max_rev, @parents) = @_;

then list-assigns

my ($self, $min_rev, $max_rev, @parents) = ($gs);

List assignments assign corresponding elements until an array or hash on the left-hand side eats all the arguments or the list of assignees is exhausted, padding the list with undef as needed. So $self gets $gs, $min_rev and $max_rev get undef, and @parents gets the empty list. It turns out that these are all valid values, and so nothing untoward happens.

If you wanted to affect the values of $min_rev et al., you would alter the call site to read

$gs->fetch(5, 9)

(it turns out @parents is ignored, so I don't know what its legal values might be).

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Nope, no assignment. I've linked directly into the source code if you want to take a look. And, as best I can tell, Perl is designed to let you commit crimes against humanity. TIMTOWTDI and all that. Damn, I miss Python.

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