From bda7bc31d6a8f90087dc7b83c00209ae9d0d3390 Mon Sep 17 00:00:00 2001
From: Tommy Bidne <tbidne@protonmail.com>
Date: Thu, 7 Nov 2024 12:48:51 +1300
Subject: [PATCH] Add mergeApps for merging multiple app AttrSets

---
 .github/workflows/ci.yaml |  5 +++
 README.md                 | 21 ++++------
 flake.nix                 |  1 +
 lib/apps.nix              | 84 +++++++++++++++++++++++++++++++++++++--
 test/flake.nix            | 11 +++++
 5 files changed, 106 insertions(+), 16 deletions(-)

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 499f99d..d67a620 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -120,3 +120,8 @@ jobs:
         run: |
           cd test
           nix run .#lint-yaml
+
+      - name: Lint merged
+        run: |
+          cd test
+          nix run .#lint-merged
diff --git a/README.md b/README.md
index 63ec2f8..1ea8c9a 100644
--- a/README.md
+++ b/README.md
@@ -126,7 +126,7 @@ Nix utility functions for haskell flakes.
 }
 ```
 
-Note that we can also merge multiple apps together, using the `mkDrv` argument (default `true`):
+Note that we can also merge multiple apps together, using the `mkDrv` argument (default `true`) and `mergeApps`:
 
 ```nix
 let
@@ -135,17 +135,12 @@ let
   formatHsNix = nix-hs-utils.format { ... mkDrv = false; ... };
   formatYaml = nix-hs-utils.format-yaml { ... mkDrv = false; ... };
 
-  # Because of 'mkDrv = false', we need to call mkShellApp ourselves.
-  format = nix-hs-utils.mkShellApp {
-    inherit pkgs;
-    name = "format";
-
-    # Now we can combine the command and inputs in a straightforward manner.
-    text = ''
-      ${formatHsNix.text}
-
-      ${formatYaml.text}
-    '';
-    runtimeInputs = formatHsNix.runtimeInputs ++ formatYaml.runtimeInputs;
+  # mergeApps takes in a list of app AttrSet and merges them together into a
+  # single app.
+  format = nix-hs-utils.mergeApps {
+    apps = [
+      (nix-hs-utils.format (compilerPkgs // pkgsMkDrv))
+      (nix-hs-utils.format-yaml pkgsMkDrv)
+    ];
   };
 ```
diff --git a/flake.nix b/flake.nix
index dd62a0e..998b080 100644
--- a/flake.nix
+++ b/flake.nix
@@ -26,6 +26,7 @@
         lint-yaml
         mkApp
         mkShellApp
+        mergeApps
         ;
     };
 }
diff --git a/lib/apps.nix b/lib/apps.nix
index f9d73c2..4465bb0 100644
--- a/lib/apps.nix
+++ b/lib/apps.nix
@@ -18,11 +18,89 @@ let
     mkApp (pkgs.writeShellApplication { inherit name text runtimeInputs; });
 
   # Returns either a shell app derivation or the set itself.
-  drvOrSet = mkDrv: s:
-    if mkDrv then mkShellApp s else s;
+  drvOrSet = mkDrv: s: if mkDrv then mkShellApp s else s;
+
+  /*
+    Merges a list of attr sets representing shell apps into a single app.
+
+    Fields:
+
+    - apps (List AttrSet):
+        NonEmpty list of AttrSet shell apps. Each app requires the 'text'
+        and 'runtimeInputs' fields.
+
+    - mkDrv (Boolean):
+        If true (default), returns a derivation i.e. the shell app. Otherwise
+        returns the merged set.
+
+    - name (String):
+        The name to use for the merged app. If null or unspecified, we take
+        the name of the __first__ app with a non-null name. At least one name
+        is required.
+
+    - pkgs (AttrSet):
+        The nixpkgs for creating the shell app. Follows the same semantics
+        as name.
+
+    Example:
+      a1 = ...
+      a2 = ...
+      a3 = ...
+
+      app = mergeApps { apps = [a1 a2 a3]; };
+
+    Type: mkLibs ::
+            List AttrSet ->
+            Boolean ->
+            Maybe String ->
+            Maybe AttrSet ->
+            (AttrSet | Derivation)
+  */
+  mergeApps =
+    {
+      apps,
+      mkDrv ? true,
+      name ? null,
+      pkgs ? null,
+    }:
+    let
+      init = {
+        inherit name pkgs;
+        text = "";
+        runtimeInputs = [ ];
+      };
+      mergeApp = acc: app: {
+        # Take the first non-null name we find. Apps are not required to
+        # have a name, but we need at least one (or top-level) name.
+        name =
+          if acc.name != null then
+            acc.name
+          else if app ? name then
+            app.name
+          else
+            acc.name;
+
+        # Same semantics as name: Take the first non-null or top-level.
+        pkgs =
+          if acc.pkgs != null then
+            acc.pkgs
+          else if app ? pkgs then
+            app.pkgs
+          else
+            acc.pkgs;
+
+        text = ''
+          ${acc.text}
+
+          ${app.text}
+        '';
+        runtimeInputs = acc.runtimeInputs ++ app.runtimeInputs;
+      };
+    in
+    drvOrSet mkDrv (builtins.foldl' mergeApp init apps);
 in
 {
-  inherit mkApp mkShellApp;
+  inherit mkApp mkShellApp mergeApps;
 
   # ShellApp that formats cabal, nix, and haskell via ormolu (default) or
   # fourmolu.
diff --git a/test/flake.nix b/test/flake.nix
index b848456..ac55297 100644
--- a/test/flake.nix
+++ b/test/flake.nix
@@ -38,6 +38,10 @@
       compilerPkgs = {
         inherit compiler pkgs;
       };
+      pkgsMkDrv = {
+        inherit pkgs;
+        mkDrv = false;
+      };
     in
     {
       packages."${system}".default = mkShell false;
@@ -87,6 +91,13 @@
         };
 
         lint-yaml = nix-hs-utils.lint-yaml { inherit pkgs; };
+
+        lint-merged = nix-hs-utils.mergeApps {
+          apps = [
+            (nix-hs-utils.lint (compilerPkgs // pkgsMkDrv))
+            (nix-hs-utils.lint-yaml pkgsMkDrv)
+          ];
+        };
       };
     };
 }