Github user tsutsu commented on a diff in the pull request:
https://github.com/apache/couchdb-couch/pull/105#discussion_r40709916
--- Diff: src/couch_stream.erl ---
@@ -51,7 +51,7 @@ open(Fd) ->
open(Fd, []).
open(Fd, Options) ->
- gen_server:start_link(couch_stream, {Fd, self(), Options}, []).
+ gen_server:start_link(couch_stream, {Fd, {self(),
erlang:get(io_priority)}, Options}, []).
--- End diff --
It's a bit of semantic idealism. Since `self()` is already being passed,
and the gen_server could theoretically extract the `io_priority` from the
passed pid() later on (using `erlang:process_info(Pid, dictionary)`, though
it's inefficient and bad form), you could say that self() "contains" the
io_priority in an OOP-theoretic sense. Effectively, the result of using
`erlang:get/1` is *derived data* of `self()`, equivalent to something like the
computed length of a list.
Frequently, you want to pass an object around along with its derived data,
in such a way that its consumers think of the composite as an ADT with the same
functionality as the original object. In a class-based language, you'd do
something silly here with a decorator proxy to enable the object you're passing
to respond to a `get_io_priority` message, but otherwise behave the same. In
Erlang, you'd either use a record (if you want to create a contract on the
shape of the ADT), or simply stick the original data together with its derived
data into a tuple.
...pragmatically, though, that's a bunch of hot air and it's an extra
structure/destructure op and could just as well be eliminated.
(It *would* have more practical value if any intermediate function calls
occurred that had just expected "an owner", where the composite `{OwnerPid,
OwnerPriority}` could "ride through" as an abstract `Owner` and then be
unwrapped only by functions that care. But none do.)
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