gen_server_virtual_time_migrate

# GenServerVirtualTimeMigrate

Move existing `GenServer` code onto the virtual clock without hand-editing every
file. The task reads your tree, shows exactly what it would change, and writes
only when you ask.

```console
$ mix virtual_time.migrate
lib/my_app/counter.ex
  line 2: use GenServer

1 file(s) would change. Re-run with --write to apply.
```

[![Hex.pm](https://img.shields.io/hexpm/v/gen_server_virtual_time_migrate.svg)](https://hex.pm/packages/gen_server_virtual_time_migrate)
[![Documentation](https://img.shields.io/badge/docs-hexdocs-blue.svg)](https://hexdocs.pm/gen_server_virtual_time_migrate)

## The family

| Package                                                                   | What it is for                                   | Add it when                                       |
| ------------------------------------------------------------------------- | ------------------------------------------------ | ------------------------------------------------- |
| [`gen_server_virtual_time`](https://hexdocs.pm/gen_server_virtual_time)   | virtual clock, `GenServer`/`gen_statem` wrappers | you test code with timers                         |
| **`gen_server_virtual_time_migrate`** (this one)                          | rewrite `GenServer` code for the virtual clock   | you already have servers and want virtual time    |
| [`actor_simulation`](https://hexdocs.pm/actor_simulation)                 | actor-system DSL, statistics, sequence diagrams  | you model parts of a system talking to each other |
| [`actor_simulation_codegen`](https://hexdocs.pm/actor_simulation_codegen) | generate runnable projects in six runtimes       | you want the same model in another language       |

This package is standalone: it only rewrites text, so it needs nothing at
runtime. The rewritten code needs `gen_server_virtual_time`.

## Install

```elixir
def deps do
  [
    {:gen_server_virtual_time, "~> 1.0.1"},
    {:gen_server_virtual_time_migrate, "~> 1.0.0", only: :dev}
  ]
end
```

## Use

```console
mix virtual_time.migrate                                  # report only
mix virtual_time.migrate --write                          # apply
mix virtual_time.migrate --check                          # exit 1 if anything would change
mix virtual_time.migrate --paths lib --paths test         # defaults to lib and test
```

A rewrite that would not parse aborts the run before anything is written, so a
`--write` either produces a compiling tree or leaves it alone.

## What it rewrites

| Before                                   | After                                                 |
| ---------------------------------------- | ----------------------------------------------------- |
| `use GenServer`                          | `use VirtualTimeGenServer`                            |
| `use GenStateMachine`                    | `use VirtualTimeGenStateMachine`                      |
| `Process.send_after(pid, :tick, 1_000)`  | `VirtualTimeGenServer.send_after(pid, :tick, 1_000)`  |
| `Process.sleep(10)`                      | `VirtualTimeGenServer.sleep(10)`                      |
| `:erlang.send_after(500, self(), :tick)` | `VirtualTimeGenServer.send_after(500, self(), :tick)` |
| `:timer.sleep(10)`                       | `VirtualTimeGenServer.sleep(10)`                      |
| `GenServer.call(MyApp.Counter, :get)`    | `VirtualTimeGenServer.call(MyApp.Counter, :get)`      |

The time calls are only rewritten inside a module that is itself converted —
that is what puts the module on the virtual clock.

## What it leaves for you

The tool never guesses, and it says what it skipped:

- `GenServer.call/2` on a pid, a variable, or a module that stays a plain
  `GenServer` — the call may still be right, the tool just cannot prove it;
- anything outside a converted module;
- `:gen_statem` timeouts, which the virtual-time state machine handles through
  its own options — see `VirtualTimeGenStateMachine`.

Comments, strings and formatting elsewhere in the file are untouched: the
rewrite is a token replacement, so `git diff` shows only the lines that changed.

## Compatibility

Input is parsed with the standard library (`Code.string_to_quoted/2`), so the
migrator needs the same Elixir as the project it runs in. The virtual-time
packages themselves keep working on Elixir 1.19 and OTP 28 as well.