feat: premix task

README.md

@@ -95,23 +95,14 @@ $ 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
+```
+
+This will generate a `blend/premix.exs` file that needs to be compiled at the top of your `mix.exs` file
+so that some mix env vars are properly set based on the `BLEND` env var before running any mix task.
 
 ##### 2. Modify your `mix.exs`.
 
@@ -155,6 +146,7 @@ $ mix blend.init         # Generate blend.exs
 $ mix blend.get          # Generate blend lockfiles
 $ mix blend.update --all # Update blend lockfiles to latest possible versions
 $ mix blend.list         # List blends
+$ mix blend.premix       # Generate premix.exs file
 ```
 
 ## License

lib/blend.ex

@@ -4,20 +4,31 @@ defmodule Blend do
   """
 
   @blend_dir "blend"
-  @blendfile_path "blend.exs"
+  @blendfile_name "blend.exs"
   @blendfile_template File.read!(Path.join(__DIR__, "blend/templates/blend.exs"))
+  @premix_file_name "premix.exs"
+  @premix_file_template File.read!(Path.join(__DIR__, "blend/templates/premix.exs"))
 
   def init do
-    case File.read(@blendfile_path) do
+    case File.read(@blendfile_name) do
       {:ok, _} ->
-        IO.puts("#{@blendfile_path} file already exists, doing nothing")
+        IO.puts("#{@blendfile_name} file already exists, doing nothing")
 
       {:error, :enoent} ->
-        File.write!(@blendfile_path, @blendfile_template)
-        IO.puts("Successfully created #{@blendfile_path} file")
+        File.write!(@blendfile_name, @blendfile_template)
+        IO.puts("Successfully created #{@blendfile_name} file")
     end
   end
 
+  def premix do
+    path = Path.join(@blend_dir, @premix_file_name)
+
+    File.mkdir_p!(@blend_dir)
+    File.write!(path, @premix_file_template)
+
+    IO.puts("Written #{path} file")
+  end
+
   def within(blend_id, fun) do
     host_project_config = Mix.Project.config()
 
@@ -49,18 +60,18 @@ defmodule Blend do
   end
 
   def blends do
-    case File.read(@blendfile_path) do
+    case File.read(@blendfile_name) do
       {:ok, contents} ->
         case Code.eval_string(contents) do
           {%{} = map, _} ->
             map
 
           _ ->
-            raise "Couldn't find a map defining your blends in #{@blendfile_path} file"
+            raise "Couldn't find a map defining your blends in #{@blendfile_name} file"
         end
 
       {:error, :enoent} ->
-        raise "Couldn't find a #{@blendfile_path} file"
+        raise "Couldn't find a #{@blendfile_name} file"
     end
   end
 end

lib/blend/templates/premix.exs

@@ -0,0 +1,22 @@
+# This file is autogenerated by blend package.
+#
+# Run `mix blend.premix` to update it's contents after
+# each blend package version update.
+
+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