expression evaluation engine for Go

 The expr package provides an engine that can compile and evaluate
 expressions. An expression is a one-liner that returns a value (mostly,
 but not limited to, booleans). It is designed for simplicity, speed and
 safety.
 .
 Features:
  - Seamless integration with Go (no need to redefine types)
  - Static typing
  - User-friendly error messages
  - Reasonable set of basic operators
  - Builtins: all, none, any, one, filter, map
  - Fast: uses bytecode virtual machine and optimizing compiler