SuccessReason: add some explanatory comments

Whenever I look at dependency calculation, it takes me a while to get my
head round the concept of influences. If what I've written here is
accurate, maybe the next person to look at this (or my future self)
will need less of a run-up.

1 files changed, 57 insertions, 0 deletions

diff --git a/IkiWiki.pm b/IkiWiki.pm
index e5da04a3b..5bb89e6aa 100644
--- a/IkiWiki.pm
+++ b/IkiWiki.pm
@@ -2564,21 +2564,48 @@ our @ISA = 'IkiWiki::SuccessReason';
 
 package IkiWiki::SuccessReason;
 
+# A blessed array-ref:
+#
+# [0]: human-readable reason for success (or, in FailReason subclass, failure)
+# [1]{""}:
+#      - if absent or false, the influences of this evaluation are "static",
+#        see the influences_static method
+#      - if true, they are dynamic (not static)
+# [1]{any other key}:
+#      the dependency types of influences, as returned by the influences method
+
 use overload (
+	# in string context, it's the human-readable reason
 	'""'	=> sub { $_[0][0] },
+	# in boolean context, SuccessReason is 1 and FailReason is 0
 	'0+'	=> sub { 1 },
+	# negating a result gives the opposite result with the same influences
 	'!'	=> sub { bless $_[0], 'IkiWiki::FailReason'},
+	# A & B = (A ? B : A) with the influences of both
 	'&'	=> sub { $_[1]->merge_influences($_[0], 1); $_[1] },
+	# A | B = (A ? A : B) with the influences of both
 	'|'	=> sub { $_[0]->merge_influences($_[1]); $_[0] },
 	fallback => 1,
 );
 
+# SuccessReason->new("human-readable reason", page => deptype, ...)
+
 sub new {
 	my $class = shift;
 	my $value = shift;
 	return bless [$value, {@_}], $class;
 }
 
+# influences(): return a reference to a copy of the hash
+# { page => dependency type } describing the pages that indirectly influenced
+# this result, but would not cause a dependency through ikiwiki's core
+# dependency logic.
+#
+# See [[todo/dependency_types]] for extensive discussion of what this means.
+#
+# influences(page => deptype, ...): remove all influences, replace them
+# with the arguments, and return a reference to a copy of the new influences.
+
 sub influences {
 	my $this=shift;
 	$this->[1]={@_} if @_;
@@ -2587,15 +2614,43 @@ sub influences {
 	return \%i;
 }
 
+# True if this result has the same influences whichever page it matches,
+# For instance, whether bar matches backlink(foo) is influenced only by
+# the set of links in foo, so its only influence is { foo => DEPEND_LINKS },
+# which does not mention bar anywhere.
+#
+# False if this result would have different influences when matching
+# different pages. For instance, when testing whether link(foo) matches bar,
+# { bar => DEPEND_LINKS } is an influence on that result, because changing
+# bar's links could change the outcome; so its influences are not the same
+# as when testing whether link(foo) matches baz.
+
 sub influences_static {
 	return ! $_[0][1]->{""};
 }
 
+# Change the influences of $this to be the influences of "$this & $other"
+# or "$this | $other".
+#
+# If both $this and $other are either successful or have influences,
+# or this is an "or" operation, the result has all the influences from
+# either of the arguments. It has dynamic influences if either argument
+# has dynamic influences.
+#
+# If this is an "and" operation, and at least one argument is a
+# FailReason with no influences, the result has no influences, and they
+# are not dynamic. For instance, link(foo) matching bar is influenced
+# by bar, but enabled(ddate) has no influences. Suppose ddate is disabled;
+# then (link(foo) and enabled(ddate)) not matching bar is not influenced by
+# bar, because it would be false however often you edit bar.
+
 sub merge_influences {
 	my $this=shift;
 	my $other=shift;
 	my $anded=shift;
 
+	# This "if" is odd because it needs to avoid negating $this
+	# or $other, which would alter the objects in-place. Be careful.
 	if (! $anded || (($this || %{$this->[1]}) &&
 	                 ($other || %{$other->[1]}))) {
 		foreach my $influence (keys %{$other->[1]}) {
@@ -2608,6 +2663,8 @@ sub merge_influences {
 	}
 }
 
+# Change $this so it is not considered to be influenced by $torm.
+
 sub remove_influence {
 	my $this=shift;
 	my $torm=shift;