Have packages option auto-detect single-package projects (#75)

Co-authored-by: Ag <aravindmallapureddy@juspay.in>
Co-authored-by: Sridhar Ratnakumar <srid@srid.ca>

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - New features
   - #63: Add `config.haskellProjects.${name}.outputs` containing all flake outputs for that project.
+  - #49: The `packages` option now autodiscovers the top-level `.cabal` file as its default value.
 - API changes
     - #37: Group `buildTools` (renamed to `tools`), `hlsCheck` and `hlintCheck` under the `devShell` submodule option; and allow disabling them all using `devShell.enable = false;` (useful if you want haskell-flake to produce just the package outputs).
     - #64: Remove hlintCheck (use [treefmt-nix](https://github.com/numtide/treefmt-nix#flake-parts) instead)

example/flake.nix

@@ -10,10 +10,7 @@
       imports = [ inputs.haskell-flake.flakeModule ];
       perSystem = { self', pkgs, ... }: {
         haskellProjects.default = {
-          packages = {
-            # You can add more than one local package here.
-            example.root = ./.; # Assumes ./example.cabal
-          };
+          # packages.example.root = ./.;  # This value is detected based on .cabal files
           # overrides = self: super: { };
           # devShell = {
           #  enable = true;  # Enabled by default

flake-module.nix

@@ -114,12 +114,53 @@ in
                     description = ''
                       Attrset of local packages in the project repository.
 
-                      Autodetected by default by looking for `.cabal` files in sub-directories.
+                      Autodiscovered by default by looking for `.cabal` files in
+                      top-level or sub-directories.
                     '';
                     default =
+                      # We look for a single *.cabal in project root. Otherwise,
+                      # look for multiple */*.cabal. Otherwise, error out.
+                      #
+                      # In future, we could just read `cabal.project`. See #76.
+                      let
+                        toplevel-cabal-paths =
+                          lib.concatMapAttrs
+                            (f: _:
+                              if lib.strings.hasSuffix ".cabal" f
+                              then { "${lib.strings.removeSuffix ''.cabal'' f}" = self; }
+                              else { }
+                            )
+                            (builtins.readDir self);
+                        subdir-cabal-paths = lib.filesystem.haskellPathsInDir self;
+                        errorNoDefault = msg:
+                          lib.asserts.assertMsg false '' 
+                              A default value for `packages` cannot be auto-detected:
+
+                                ${msg}
+                              You must manually specify the `packages` option.
+                            '';
+                        cabal-paths =
+                          if toplevel-cabal-paths != { }
+                          then
+                            let cabalNames = lib.attrNames toplevel-cabal-paths;
+                            in if builtins.length cabalNames > 1
+                            then
+                              errorNoDefault ''
+                                More than one .cabal file found in project root:
+
+                                  - ${lib.concatStringsSep ".cabal\n  - " cabalNames}.cabal
+                              ''
+                            else
+                              toplevel-cabal-paths
+                          else if subdir-cabal-paths != { }
+                          then
+                            subdir-cabal-paths
+                          else
+                            errorNoDefault "No .cabal file found.";
+                      in
                       lib.mapAttrs
                         (_: value: { root = value; })
-                        (lib.filesystem.haskellPathsInDir self);
+                        cabal-paths;
                     defaultText = lib.literalMD "autodiscovered by reading `self` files.";
                   };
                   devShell = mkOption {

runtest.sh

@@ -13,9 +13,17 @@ else
 fi
 
 FLAKE=$(pwd)
-cd ./test
 
-# First, build the flake.
+# A Nix bug causes incorrect self when in a sub-flake.
+# https://github.com/NixOS/nix/issues/7263
+# Workaround: copy ./test somewhere outside of this Git repo.
+TESTDIR=$(mktemp -d)
+trap 'rm -fr "$TESTDIR"' EXIT
+cp -r ./test/* "$TESTDIR"
+cd "$TESTDIR"
+pwd
+
+# First, build the flake
 logHeader "Testing nix build"
 nix build --override-input haskell-flake path:${FLAKE}
 # Run the devshell test script in a nix develop shell.

test/flake.nix

@@ -24,10 +24,6 @@
       ];
       perSystem = { self', pkgs, ... }: {
         haskellProjects.default = {
-          packages = {
-            # You can add more than one local package here.
-            haskell-flake-test.root = ./.; # Assumes ./haskell-flake-test.cabal
-          };
           overrides = self: super: {
             # Custom library overrides (here, "foo" comes from a flake input)
             foo = self.callCabal2nix "foo" (inputs.haskell-multi-nix + /foo) { };