Thanks for the article. There's a class of problems that I wouldn't know how to solve using the constrains you propose.

Given

    { a: { b: { c: 4 } }, d: 2 }

return

    { 'a.b.c': 4, d: 2 }

My go to solution uses mutable state, self-recursion, and intermediate variables:

    function flatten(source) {
      var result = {}
      _flatten(source, null, result)
      return result
    }

where `_flatten(source, prefix, result)` is self-recursive (`prefix` accumulates the keys) and populates `result[prefix]=value` when value is a string. `result` is conceptually an IO monad that's passed along.

I guess the general problem would be traverse a tree of arbitrary depth and accumulate structured data on the go. Prominent examples would be compilers.

Any hints on where to start?