feat: premix task

README.md

@@ -95,22 +95,10 @@ $ mix test
 A more permanent configuration for running mix tasks in the context of a blend lockfile with a simple env var
 can be acomplished by customizing your `mix.exs` a bit, with the following steps.
 
-##### 1. Create a new file `blend/premix.exs` with the following contents:
+##### 1. Create a new file `blend/premix.exs` with the following command:
 
-```elixir
-# blend/premix.exs
-
-maybe_put_env = fn varname, value ->
-  System.put_env(varname, System.get_env(varname, value))
-end
-
-blend = System.get_env("BLEND")
-
-if blend && String.length(blend) > 0 do
-  maybe_put_env.("MIX_LOCKFILE", "blend/#{blend}.mix.lock")
-  maybe_put_env.("MIX_DEPS_PATH", "blend/deps/#{blend}")
-  maybe_put_env.("MIX_BUILD_ROOT", "blend/_build/#{blend}")
-end
+```
+$ mix blend.premix
 ```
 
 ##### 2. Modify your `mix.exs`.

lib/blend.ex

@@ -6,6 +6,8 @@ defmodule Blend do
   @blend_dir "blend"
   @blendfile_path "blend.exs"
   @blendfile_template File.read!(Path.join(__DIR__, "blend/templates/blend.exs"))
+  @premix_file_path Path.join(@blend_dir, "premix.exs")
+  @premix_file_template File.read!(Path.join(__DIR__, "blend/templates/premix.exs"))
 
   def init do
     case File.read(@blendfile_path) do
@@ -18,6 +20,11 @@ defmodule Blend do
     end
   end
 
+  def premix do
+    File.write!(@premix_file_path, @premix_file_template)
+    IO.puts("Written #{@premix_file_path} file")
+  end
+
   def within(blend_id, fun) do
     host_project_config = Mix.Project.config()
 

lib/blend/templates/premix.exs

@@ -0,0 +1,17 @@
+maybe_put_env = fn varname, value ->
+  System.put_env(varname, System.get_env(varname, value))
+end
+
+existing_blend = fn name ->
+  Code.eval_file("blend.exs")
+  |> elem(0)
+  |> Map.fetch!(String.to_atom(name))
+end
+
+blend = System.get_env("BLEND")
+
+if blend && String.length(blend) > 0 && existing_blend.(blend) do
+  maybe_put_env.("MIX_LOCKFILE", "blend/#{blend}.mix.lock")
+  maybe_put_env.("MIX_DEPS_PATH", "blend/deps/#{blend}")
+  maybe_put_env.("MIX_BUILD_ROOT", "blend/_build/#{blend}")
+end

lib/mix/tasks/blend/premix.ex

@@ -0,0 +1,10 @@
+defmodule Mix.Tasks.Blend.Premix do
+  use Mix.Task
+
+  @shortdoc "Generates premix.exs file"
+
+  @impl true
+  def run(_args) do
+    Blend.premix()
+  end
+end