  Modularity and efficiency are often contradicting requirements, such that
programers have to trade one for the other. We analyze this dilemma in the
context of programs operating on collections. Performance-critical code using
collections need often to be hand-optimized, leading to non-modular, brittle,
and redundant code. In principle, this dilemma could be avoided by automatic
collection-specific optimizations, such as fusion of collection traversals,
usage of indexing, or reordering of filters. Unfortunately, it is not obvious
how to encode such optimizations in terms of ordinary collection APIs, because
the program operating on the collections is not reified and hence cannot be
analyzed.
  We propose SQuOpt, the Scala Query Optimizer--a deep embedding of the Scala
collections API that allows such analyses and optimizations to be defined and
executed within Scala, without relying on external tools or compiler
extensions. SQuOpt provides the same "look and feel" (syntax and static typing
guarantees) as the standard collections API. We evaluate SQuOpt by
re-implementing several code analyses of the Findbugs tool using SQuOpt, show
average speedups of 12x with a maximum of 12800x and hence demonstrate that
SQuOpt can reconcile modularity and efficiency in real-world applications.
