Don't infer Paths_-module with spec-version: 0.36.0 or later

.ghci

@@ -1 +1 @@
-:set -XHaskell2010 -fno-warn-incomplete-uni-patterns -DTEST -isrc -itest -i./dist-newstyle/build/x86_64-linux/ghc-9.6.2/hpack-0.35.3/build/autogen/
+:set -XHaskell2010 -Wredundant-constraints -fno-warn-incomplete-uni-patterns -DTEST -isrc -itest -i./dist-newstyle/build/x86_64-linux/ghc-9.6.2/hpack-0.36.0/build/autogen

CHANGELOG.md

@@ -1,3 +1,6 @@
+## Changes in 0.36.0
+ - Don't infer `Paths_`-module with `spec-version: 0.36.0` or later
+
 ## Changes in 0.35.5
  - Add (undocumented) `list` command
 

README.md

@@ -47,6 +47,8 @@ at the Singapore Haskell meetup: http://typeful.net/talks/hpack
       * [Examples](#examples)
       * [Documentation](#documentation)
          * [Handling of Paths_ modules](#handling-of-paths_-modules)
+            * [Modern behavior](#modern-behavior)
+            * [Legacy behavior](#legacy-behavior)
          * [Quick-reference](#quick-reference)
             * [Top-level fields](#top-level-fields)
             * [cabal-version](#cabal-version)
@@ -70,18 +72,41 @@ at the Singapore Haskell meetup: http://typeful.net/talks/hpack
       * [Stack support](#stack-support)
       * [Binaries for use on Travis CI](#binaries-for-use-on-travis-ci)
 
-<!-- Added by: sol, at: Fri 19 Feb 2021 10:31:47 PM +07 -->
+<!-- Added by: sol, at: Mon Sep 18 11:40:17 AM +07 2023 -->
 
 <!--te-->
 
 ### Handling of `Paths_` modules
 
-Cabal generates a `Paths_` module for every package.  By default Hpack adds
-that module to `other-modules` when generating a `.cabal` file.  This is
-sometimes useful and most of the time not harmful.
+Cabal generates a `Paths_` module for every package.  How exactly Hpack behaves
+in regards to that module depends on the value of the `spec-version` field.
 
-However, there are situations when this can lead to compilation errors (e.g
-when using a custom `Prelude`).
+If the `spec-version` is explicitly specified and at least `0.36.0` the modern
+behavior is used, otherwise Hpack falls back to the legacy behavior.
+
+To use the modern behavior, require at least
+```yaml
+spec-version: 0.36.0
+```
+in your `package.yaml`.
+
+#### Modern behavior
+
+If you want to use the `Paths_` module for a component, you have to explicitly
+specify it under `generated-other-modules`.
+
+***Example:***
+
+```yaml
+library:
+  source-dirs: src
+  generated-other-modules: Paths_name # substitute name with the package name
+```
+
+#### Legacy behavior
+
+For historic reasons Hpack adds the `Paths_` module to `other-modules` when
+generating a `.cabal` file.
 
 To prevent Hpack from adding the `Paths_` module to `other-modules` add the
 following to `package.yaml`:

cabal.project

@@ -3,3 +3,5 @@ packages:
 
 package hpack
   ghc-options: -Werror
+
+tests: True

package.yaml

@@ -1,5 +1,6 @@
+spec-version: 0.36.0
 name: hpack
-version: 0.35.5
+version: 0.36.0
 synopsis: A modern format for Haskell packages
 description: See README at <https://github.com/sol/hpack#readme>
 author: Simon Hengel <[email protected]>
@@ -50,6 +51,7 @@ library:
     - Hpack.Render
     - Hpack.Yaml
     - Hpack.Error
+  generated-other-modules: Paths_hpack
 
 executable:
   main: Main.hs
@@ -64,6 +66,7 @@ tests:
     source-dirs:
       - test
       - src
+    generated-other-modules: Paths_hpack
     dependencies:
       - hspec == 2.*
       - QuickCheck

src/Hpack/Config.hs

@@ -99,6 +99,8 @@ import           Data.Text.Encoding (decodeUtf8)
 import           Data.Scientific (Scientific)
 import           System.Directory
 import           System.FilePath
+import           Control.Monad.State (MonadState, StateT, evalStateT)
+import qualified Control.Monad.State as State
 import           Control.Monad.Writer (MonadWriter, WriterT, runWriterT, tell)
 import           Control.Monad.Except
 import           Data.Version (Version, makeVersion, showVersion)
@@ -639,7 +641,7 @@ liftIOEither action = liftIO action >>= liftEither
 
 type FormatYamlParseError = FilePath -> Yaml.ParseException -> String
 
-decodeYaml :: (FromValue a, MonadIO m, Warnings m, Errors m) => FormatYamlParseError -> FilePath -> m a
+decodeYaml :: (FromValue a, MonadIO m, Warnings m, Errors m, State m) => FormatYamlParseError -> FilePath -> m a
 decodeYaml formatYamlParseError file = do
   (warnings, a) <- liftIOEither $ first (ParseError . formatYamlParseError file) <$> Yaml.decodeYamlWithParseError file
   tell warnings
@@ -668,11 +670,12 @@ readPackageConfig options = first (formatHpackError $ decodeOptionsProgramName o
 
 type Errors = MonadError HpackError
 type Warnings = MonadWriter [String]
+type State = MonadState SpecVersion
 
-type ConfigM m = WriterT [String] (ExceptT HpackError m)
+type ConfigM m = StateT SpecVersion (WriterT [String] (ExceptT HpackError m))
 
-runConfigM :: ConfigM m a -> m (Either HpackError (a, [String]))
-runConfigM = runExceptT . runWriterT
+runConfigM :: Monad m => ConfigM m a -> m (Either HpackError (a, [String]))
+runConfigM = runExceptT . runWriterT . (`evalStateT` NoSpecVersion)
 
 readPackageConfigWithError :: DecodeOptions -> IO (Either HpackError DecodeResult)
 readPackageConfigWithError (DecodeOptions _ file mUserDataDir readValue formatYamlParseError) = fmap (fmap addCabalFile) . runConfigM $ do
@@ -902,15 +905,16 @@ determineCabalVersion inferredLicense pkg@Package{..} = (
 sectionAll :: Monoid b => (Section a -> b) -> Section a -> b
 sectionAll f sect = f sect <> foldMap (foldMap $ sectionAll f) (sectionConditionals sect)
 
-decodeValue :: (FromValue a, Warnings m, Errors m) => FilePath -> Value -> m a
+decodeValue :: (FromValue a, State m, Warnings m, Errors m) => FilePath -> Value -> m a
 decodeValue file value = do
   (r, unknown, deprecated) <- liftEither $ first (DecodeValueError file) (Config.decodeValue value)
   case r of
     UnsupportedSpecVersion v -> do
       throwError $ HpackVersionNotSupported file v Hpack.version
-    SupportedSpecVersion a -> do
+    SupportedSpecVersion v a -> do
       tell (map formatUnknownField unknown)
       tell (map formatDeprecatedField deprecated)
+      State.modify $ max v
       return a
   where
     prefix :: String
@@ -922,14 +926,20 @@ decodeValue file value = do
     formatDeprecatedField :: (String, String) -> String
     formatDeprecatedField (name, substitute) = prefix <> name <> " is deprecated, use " <> substitute <> " instead"
 
-data CheckSpecVersion a = SupportedSpecVersion a | UnsupportedSpecVersion Version
+data SpecVersion = NoSpecVersion | SpecVersion Version
+  deriving (Eq, Show, Ord)
+
+toSpecVersion :: Maybe ParseSpecVersion -> SpecVersion
+toSpecVersion = maybe NoSpecVersion (SpecVersion . unParseSpecVersion)
+
+data CheckSpecVersion a = SupportedSpecVersion SpecVersion a | UnsupportedSpecVersion Version
 
 instance FromValue a => FromValue (CheckSpecVersion a) where
   fromValue = withObject $ \ o -> o .:? "spec-version" >>= \ case
     Just (ParseSpecVersion v) | Hpack.version < v -> return $ UnsupportedSpecVersion v
-    _ -> SupportedSpecVersion <$> fromValue (Object o)
+    v -> SupportedSpecVersion (toSpecVersion v) <$> fromValue (Object o)
 
-newtype ParseSpecVersion = ParseSpecVersion Version
+newtype ParseSpecVersion = ParseSpecVersion {unParseSpecVersion :: Version}
 
 instance FromValue ParseSpecVersion where
   fromValue value = do
@@ -1079,7 +1089,7 @@ toPackage formatYamlParseError userDataDir dir =
         setLanguage = (mempty { commonOptionsLanguage = Alias . Last $ Just (Just language) } <>)
 
 expandDefaultsInConfig
-  :: (MonadIO m, Warnings m, Errors m) =>
+  :: (MonadIO m, Warnings m, Errors m, State m) =>
      FormatYamlParseError
   -> FilePath
   -> FilePath
@@ -1088,7 +1098,7 @@ expandDefaultsInConfig
 expandDefaultsInConfig formatYamlParseError userDataDir dir = bitraverse (expandGlobalDefaults formatYamlParseError userDataDir dir) (expandSectionDefaults formatYamlParseError userDataDir dir)
 
 expandGlobalDefaults
-  :: (MonadIO m, Warnings m, Errors m) =>
+  :: (MonadIO m, Warnings m, Errors m, State m) =>
      FormatYamlParseError
   -> FilePath
   -> FilePath
@@ -1098,7 +1108,7 @@ expandGlobalDefaults formatYamlParseError userDataDir dir = do
   fmap (`Product` Empty) >>> expandDefaults formatYamlParseError userDataDir dir >=> \ (Product c Empty) -> return c
 
 expandSectionDefaults
-  :: (MonadIO m, Warnings m, Errors m) =>
+  :: (MonadIO m, Warnings m, Errors m, State m) =>
      FormatYamlParseError
   -> FilePath
   -> FilePath
@@ -1121,7 +1131,7 @@ expandSectionDefaults formatYamlParseError userDataDir dir p@PackageConfig{..} =
     }
 
 expandDefaults
-  :: forall a m. (MonadIO m, Warnings m, Errors m) =>
+  :: forall a m. (MonadIO m, Warnings m, Errors m, State m) =>
      (FromValue a, Monoid a)
   => FormatYamlParseError
   -> FilePath
@@ -1169,7 +1179,7 @@ toExecutableMap name executables mExecutable = do
 
 type GlobalOptions = CommonOptions CSources CxxSources JsSources Empty
 
-toPackage_ :: (MonadIO m, Warnings m) => FilePath -> Product GlobalOptions (PackageConfig CSources CxxSources JsSources) -> m (Package, String)
+toPackage_ :: (MonadIO m, Warnings m, State m) => FilePath -> Product GlobalOptions (PackageConfig CSources CxxSources JsSources) -> m (Package, String)
 toPackage_ dir (Product g PackageConfig{..}) = do
   executableMap <- toExecutableMap packageName_ packageConfigExecutables packageConfigExecutable
   let
@@ -1377,7 +1387,7 @@ removeConditionalsThatAreAlwaysFalse sect = sect {
   where
     p = (/= CondBool False) . conditionalCondition
 
-inferModules :: MonadIO m =>
+inferModules :: (MonadIO m, State m) =>
      FilePath
   -> String
   -> (a -> [Module])
@@ -1386,10 +1396,19 @@ inferModules :: MonadIO m =>
   -> ([Module] -> a -> b)
   -> Section a
   -> m (Section b)
-inferModules dir packageName_ getMentionedModules getInferredModules fromData fromConditionals = fmap removeConditionalsThatAreAlwaysFalse . traverseSectionAndConditionals
-  (fromConfigSection fromData [pathsModuleFromPackageName packageName_])
-  (fromConfigSection (\ [] -> fromConditionals) [])
-  []
+inferModules dir packageName_ getMentionedModules getInferredModules fromData fromConditionals sect_ = do
+  specVersion <- State.get
+  let
+    pathsModule :: [Module]
+    pathsModule = case specVersion of
+      SpecVersion v | v >= makeVersion [0,36,0] -> []
+      _ -> [pathsModuleFromPackageName packageName_]
+
+  removeConditionalsThatAreAlwaysFalse <$> traverseSectionAndConditionals
+    (fromConfigSection fromData pathsModule)
+    (fromConfigSection (\ [] -> fromConditionals) [])
+    []
+    sect_
   where
     fromConfigSection fromConfig pathsModule_ outerModules sect@Section{sectionData = conf} = do
       modules <- liftIO $ listModules dir sect
@@ -1400,7 +1419,7 @@ inferModules dir packageName_ getMentionedModules getInferredModules fromData fr
         r = fromConfig pathsModule inferableModules conf
       return (outerModules ++ getInferredModules r, r)
 
-toLibrary :: MonadIO m => FilePath -> String -> Section LibrarySection -> m (Section Library)
+toLibrary :: (MonadIO m, State m) => FilePath -> String -> Section LibrarySection -> m (Section Library)
 toLibrary dir name =
     inferModules dir name getMentionedLibraryModules getLibraryModules fromLibrarySectionTopLevel fromLibrarySectionInConditional
   where
@@ -1444,7 +1463,7 @@ getMentionedExecutableModules :: ExecutableSection -> [Module]
 getMentionedExecutableModules (ExecutableSection (Alias (Last main)) otherModules generatedModules)=
   maybe id (:) (toModule . Path.fromFilePath <$> main) $ fromMaybeList (otherModules <> generatedModules)
 
-toExecutable :: MonadIO m => FilePath -> String -> Section ExecutableSection -> m (Section Executable)
+toExecutable :: (MonadIO m, State m) => FilePath -> String -> Section ExecutableSection -> m (Section Executable)
 toExecutable dir packageName_ =
     inferModules dir packageName_ getMentionedExecutableModules getExecutableModules fromExecutableSection (fromExecutableSection [])
   . expandMain

test/EndToEndSpec.hs

@@ -77,6 +77,15 @@ spec = around_ (inTempDirectoryNamed "my-package") $ do
         default-language: Haskell2010
         |]
 
+      context "when spec-version is >= 0.36.0" $ do
+        it "does not add Paths_" $ do
+          [i|
+          spec-version: 0.36.0
+          library: {}
+          |] `shouldRenderTo` library [i|
+          default-language: Haskell2010
+          |]
+
       context "when cabal-version is >= 2" $ do
         it "adds Paths_ to autogen-modules" $ do
           [i|