Comment by logicchains

Something I find quite interesting about Nim is write tracking: http://nim-lang.org/blog/writetracking.html. It uses the language's effect system, and allows not only specifying for instance that a function is referentially transparent, but also specifying how an impure function accesses or modifies global state.

An example from the above link:

    var gId = 0

    proc genId(): natural {.writes: [gId].} =
      gId += 1
      return gId

To quote the article: "Here the effect systems shows its strength: Imagine there was a genId2 that writes to some other global variable; then genId and genId2 can be executed in parallel even though they are not free of side effects!"

The effect system can also be used for exception tracking, specifying what kind of exceptions are function may throw. It can moreover be extended to work with user-defined effects.