Design Rationale

Selectors are not Queries

Since both a Query and a Selector work to limit the set of all Sack’s packages to a subset, it can be suggested the two concepts should be the same and e.g. Queries should be used for Goal specifications instead of Selectors:

// create sack, goal, ...
HyQuery q = hy_query_create(sack);
hy_query_filter(q, HY_PKG_NAME, HY_EQ, "anaconda")

This arrangment was in fact used in hawkey prior to version 0.3.0, just because Queries looked like a convenient structure to hold this kind of information. It was unfortunately confusing for the programmers: notice how evaluating the Query q would generally produce several packages (anaconda for different architectures and then different versions) but somehow when the same Query is passed into the goal methods it always results in up to one pacakge selected for the operation. This is a principal discrepancy. Further, Query is universal and allows one to limit the package set with all sorts of criteria, matched in different ways (substrings, globbing, set operation) while Selectors only support few. Finally, while a fresh Query with no filters applied corresponds to all packages of the Sack, a fresh Selector with no limits set is of no meaning.

An alternative to introducing a completely different concept was adding a separate constructor function for Query, one that would from the start designate the Query to only accept settings compatible with its purpose of becoming the selecting element in a Goal operation (in Python this would probably be implemented as a subclass of Query). But that would break client’s assumptions about Query (the unofficial C++ FAQ takes up the topic).

Implementation note: Selectors reflect the kind of specifications that can be directly translated into Libsolv jobs, without actually searching for a concrete package to put there. In other words, Selectors are specifically designed not to iterate over the package data (with exceptions, like glob matching) like Queries do. While Hawkey mostly aims to hide any twists and complexities of the underlying library, in this case the combined reasons warrant a concession.