Make package file discovery more robust

snack-lib/default.nix

@@ -16,9 +16,10 @@ with (callPackage ./lib.nix {});
 with (callPackage ./modules.nix {});
 with (callPackage ./module-spec.nix {});
 with (callPackage ./package-spec.nix {});
-with (callPackage ./hpack.nix {});
 
-let
+with rec
+{
+  hpack = callPackage ./hpack.nix { inherit pkgDescriptionsFromPath; };
 
   # Derivation that creates a binary in a 'bin' folder.
   executable = packageFile:
@@ -119,31 +120,66 @@ let
         in modSpecs.${mainModName};
     in mainModSpec;
 
-  # Get a list of package specs from a file (.nix or .yaml)
-  specsFromPackageFile = packageFile:
-    let
-      basename = builtins.baseNameOf packageFile;
-      components = pkgs.lib.strings.splitString "." basename;
-      ext =
-        if pkgs.lib.length components <= 1
-        then abort ("File " ++ packageFile ++ " does not have an extension")
-        else pkgs.lib.last components;
-      fromNix = [(mkPackageSpec (import packageFile))];
-      fromHPack = builtins.map mkPackageSpec (pkgSpecsFromHPack packageFile);
-      specs =
+  # Get a list of package descriptions from a path
+  # This can be
+  #  - a path, relative or absolute, to a directory that contains either a
+  #     package.yaml or a package.nix
+  #  - a path, relative or absolute, to a file with either .nix or .yaml or
+  #     .yml extension
+
+  pkgDescriptionsFromPath =
+    with rec
+    {
+      pkgDescriptionsFromFile = packageFile:
+        with rec
+        {
+          basename = builtins.baseNameOf packageFile;
+          components = pkgs.lib.strings.splitString "." basename;
+          ext =
+            if pkgs.lib.length components <= 1
+            then abort ("File " ++ packageFile ++ " does not have an extension")
+            else pkgs.lib.last components;
+          fromNix = [(import packageFile)];
+          fromHPack = hpack.pkgDescriptionsFromHPack packageFile;
+        };
         if ext == "nix" then fromNix
         else if ext == "yaml" then fromHPack
         else if ext == "yml" then fromHPack
-        else abort ("Unknown extension " ++ ext ++ " of file " ++ packageFile);
-    in specs;
+        else abort ("Unknown extension " ++ ext ++ " of file " ++ packagePath);
+      pkgDescriptionsFromDir = packageDir:
+        with rec
+        { dirContent = builtins.readDir packageDir;
+          hasPackageYaml = builtins.hasAttr "package.yaml" dirContent;
+          hasPackageNix = builtins.hasAttr "package.nix" dirContent;
+        };
+        if hasPackageYaml && hasPackageNix
+          then abort "Found both package.yaml and package.nix in ${packageDir}"
+        else if ! (hasPackageYaml || hasPackageNix)
+          then abort "Couldn't find package.yaml or package.nix in ${packageDir}"
+        else if hasPackageYaml
+          then pkgDescriptionsFromFile
+            "${builtins.toString packageDir}/package.yaml"
+        else  pkgDescriptionsFromFile
+            "${builtins.toString packageDir}/package.nix";
+    };
 
-in
-  {
-    inherit
-    inferBuild
-    inferGhci
-    buildAsExecutable
-    buildAsLibrary
-    executable
-    ;
-  }
+    packagePath:
+    with { pathType = pkgs.lib.pathType packagePath ; } ;
+    if pathType == "directory"
+      then pkgDescriptionsFromDir packagePath
+    else if pathType == "regular"
+      then pkgDescriptionsFromFile packagePath
+    else abort "Don't know how to load package path of type ${pathType}";
+
+    specsFromPackageFile = packagePath:
+      map mkPackageSpec (pkgDescriptionsFromPath packagePath);
+};
+{
+  inherit
+  inferBuild
+  inferGhci
+  buildAsExecutable
+  buildAsLibrary
+  executable
+  ;
+}

snack-lib/hpack.nix

@@ -1,4 +1,11 @@
-{ lib, glibcLocales, callPackage, writeText, runCommand, haskellPackages }:
+{ lib
+, glibcLocales
+, callPackage
+, writeText
+, runCommand
+, haskellPackages
+, pkgDescriptionsFromPath
+}:
 
 with (callPackage ./lib.nix {});
 with (callPackage ./modules.nix {});
@@ -22,10 +29,7 @@ let
       in builtins.fromJSON json;
 in rec
 {
-  # Returns an attribute set with two fields:
-  #  - library: a package spec
-  #  - executable: an attr set of executable name to package spec
-  pkgSpecsFromHPack = packageYaml:
+  pkgDescriptionsFromHPack = packageYaml:
     let
         package = fromYAML (builtins.readFile packageYaml);
         base = builtins.dirOf packageYaml;
@@ -70,11 +74,10 @@ in rec
                   # This is extremely brittle:
                   #   - there could be more than one package
                   #   - this needs to make sure it only picks libraries
-                  #   - it only works with "package.yaml"
                   [
                     (lib.head
-                      ( pkgSpecsFromHPack
-                        ("${builtins.toString base}/${x}/package.yaml")
+                      ( pkgDescriptionsFromPath
+                        ("${builtins.toString base}/${x}")
                       )
                     )
                   ]

tests/packages-2/Main.hs

@@ -1,3 +1,4 @@
 import Lib (fromLib)
+import Lib2 (fromLib2)
 
-main = putStrLn fromLib
+main = putStrLn (fromLib <> fromLib2)

tests/packages-2/lib/Lib.hs

@@ -1,4 +1,4 @@
 module Lib (fromLib) where
 
 fromLib :: String
-fromLib = "hello"
+fromLib = "hel"

tests/packages-2/lib/package.yaml

@@ -1,4 +1,4 @@
-name: snack-packages-2
+name: snack-packages-2-lib
 
 # NOTE: should not fail if library is empty
 #  For some reason, Nix returns null as opposed to {}

tests/packages-2/lib2/Lib2.hs

@@ -0,0 +1,4 @@
+module Lib2 (fromLib2) where
+
+fromLib2 :: String
+fromLib2 = "lo"

tests/packages-2/lib2/package.yaml

@@ -0,0 +1,6 @@
+name: snack-packages-2-lib2
+
+# NOTE: should not fail if library is empty
+#  For some reason, Nix returns null as opposed to {}
+library:
+    source-dirs: .

tests/packages-2/package.yaml

@@ -4,3 +4,4 @@ executable:
     main: Main.hs
     dependencies:
         - ./lib
+        - ./lib2/package.yaml