From simple to parallel
=======================

Single-threaded code have some advantages:

* Can modify data in place without issues
* Does not need synchronization
* Overall, it can be lightweight and simple

Can code be made to be usable in both simple single-threaded code
as well as in multi-threaded code?

(Note that single-threaded includes some forms of "shared nothing"
parallelism).

In-place mutation vs. copy vs. specialized multi-threaded code
--------------------------------------------------------------

(The latter might be using some kind of multi-threading capable data structure)

Let's say that there is a function that appends "=true" to a string if it does
not already end with "=true".

This can be done in several ways:

* In-place modification, if the string has sufficient capacity.
* Allocating a new string and writing the result string there.
* Using a thread-safe refcount to select the best strategy.
* Using some kind of data structure that allows this operation
  to work without copying. E.g. a chain of delta encoded updates
  to the string. This could be done in a lock-free/opportunistic way:
    - Find the last delta entry of the string.
    - Parse the string, check if it ends with "=true".
    - Generate a "delta entry" to append.
    - Atomically swap the pointer in the last delta entry
      from null to the newly generated delta entry.
    - If the swap operation fails, restart the process.

The idea
--------

Can we generate "combined single/multi-threaded" code from the same source
code, and have the compiler (and/or runtime) select the appropriate code
variant?

Related: Combined code for mutator methods:
1. pre-allocated result (with mandatory usage)
2. pre-allocated result (with optional usage, i.e. input lifetime is at least as long)
3. arena-allocated result
4. shallow-ref in-place modification (with existing refs)
5. deep-copy in-place modification (with copies of refs)

For example, given source code like this:

    func String.append_if_absent(String suffix) -> String
    {
        if this.ends_with(suffix) {
            return this
        } else {
            return .concat(this, suffix)
        }
    }

It might be transformed into:

    func String.append_if_absent(arena, ref var String placement, String suffix) -> String
    {
        String result
        transaction_start(arena, .[this])
        do {
            if this.ends_with(suffix) {
                result = .copy_if_needed(arena, placement, this)
            } else {
                # .concat selects the requested method 1-5
                result = .concat(arena, placement, this, suffix)
            }
        } while not transaction_commit(arena)
        return result
    }

Apparently. something similar is done by Swift already and is called "value semantics".
- ...and apparently, it can still cause unintended behavior when a value
  gets copied and then silently discarded.