From dc2ce818b854eaa1b4c7ada7ec1c2b921c8d395c Mon Sep 17 00:00:00 2001
From: Jake Hamilton <jake.hamilton@hey.com>
Date: Mon, 17 Jun 2024 03:59:11 -0700
Subject: [PATCH] feat: apply dag defaults

---
 lib/src/dag/default.nix | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib/src/dag/default.nix b/lib/src/dag/default.nix
index a904740..15bdb6b 100644
--- a/lib/src/dag/default.nix
+++ b/lib/src/dag/default.nix
@@ -59,7 +59,34 @@ lib: {
     };
 
     apply = {
-      # defaults = graph: defaults:
+      ## Apply a set of defaults to a graph. This will ensure that missing entries are added
+      ## and any entries that do exist are given the appropriate `before` and `after` values.
+      ##
+      ## @type Dag a -> Dag a -> Dag a
+      defaults = graph: defaults: let
+        result =
+          builtins.mapAttrs
+          (
+            name: entry:
+              if defaults ? ${name}
+              then
+                if builtins.isString entry
+                then {
+                  value = entry;
+                  before = defaults.${name}.before or [];
+                  after = defaults.${name}.after or [];
+                }
+                else
+                  entry
+                  // {
+                    before = (entry.before or []) ++ (defaults.${name}.before or []);
+                    after = (entry.after or []) ++ (defaults.${name}.after or []);
+                  }
+              else entry
+          )
+          graph;
+      in
+        defaults // result;
     };
 
     ## Map over the entries in a DAG and modify their values.