Folly (acronymed loosely after Facebook Open Source Library) is a library of
C++14 components designed with practicality and efficiency in mind. Folly
contains a variety of core library components used extensively at Facebook. In
particular, it's often a dependency of Facebook's other open source C++ efforts
and place where those projects can share code.

It complements (as opposed to competing against) offerings such as Boost and of
course std. In fact, we embark on defining our own component only when something
we need is either not available, or does not meet the needed performance
profile. We endeavor to remove things from folly if or when std or Boost
obsoletes them.

Performance concerns permeate much of Folly, sometimes leading to designs that
are more idiosyncratic than they would otherwise be (see e.g. PackedSyncPtr.h,
SmallLocks.h). Good performance at large scale is a unifying theme in all of
Folly.