Hello everyone i'am looking for method to change field types and convert its values during this.  

I mean something like that:

```
def up do
  alter table(:users) do
    modify :role, :integer, default: fragment("convertion_function")
  end
end

```

I know [`Ecto.Migration#modify/3`](https://hexdocs.pm/ecto/Ecto.Migration.html#modify/3) function gets *`&fragment/1`* argument. But it gets only one argument.  

Does anyone know if it possible to pass current value to *`&fragment/1`* function?  

Or maybe anyone know better way to do that?