# `mix virtual_time.migrate`
[🔗](https://github.com/d-led/gen_server_virtual_time/blob/v1.1.1/packages/gen_server_virtual_time_migrate/lib/mix/tasks/virtual_time.migrate.ex#L1)

Rewrites `GenServer` and `GenStateMachine` modules to their virtual-time
equivalents, reporting first and writing only when asked.

## Usage

    mix virtual_time.migrate                       # what would change, and what needs a human
    mix virtual_time.migrate --write               # apply it
    mix virtual_time.migrate --check               # exit 1 when something would change (CI)
    mix virtual_time.migrate --paths lib --paths test

## What it rewrites

  * `use GenServer` → `use VirtualTimeGenServer`, options untouched;
    `use GenStateMachine` → `use VirtualTimeGenStateMachine`;
  * inside a converted module, `Process.send_after/3`, `Process.sleep/1` and
    `:timer.sleep/1` → the `VirtualTimeGenServer` equivalents, which is what
    puts the module on the virtual clock;
  * `GenServer.call/cast/stop/reply/start_link/start/child_spec` when the
    target is a literal module this tree converts, or `__MODULE__`.

It never guesses: a call on a pid or a variable is reported and left alone, a
call to a module that stays a plain `GenServer` is not touched, and
`:erlang.send_after/3` is reported rather than rewritten because its delay is
the first argument where the virtual-time call takes it last. A file
that does not parse is reported and skipped, and a rewrite that would not
parse aborts the whole run without writing anything.

The rewrite is a token replacement, not a reformat: the rest of the file is
byte for byte what it was, so the diff shows only what changed.

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*Consult [api-reference.md](api-reference.md) for complete listing*