Omit false conds

README.md

@@ -35,6 +35,25 @@ at the Singapore Haskell meetup: http://typeful.net/talks/hpack
 
 ## Documentation
 
+### 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.
+
+However, there are situations when this can lead to compilation errors (e.g
+when using a custom `Prelude`).
+
+To prevent Hpack from adding the `Paths_` module to `other-modules` add the
+following to `package.yaml`:
+
+```yaml
+library:
+  when:
+  - condition: false
+    other-modules: Paths_name # substitute name with the package name
+```
+
 ### Quick-reference
 
 #### Top-level fields
@@ -376,7 +395,6 @@ Conditionals with an else branch:
 - Must have a `condition` field
 - Must have a `then` field, itself an object containing any number of other fields
 - Must have a `else` field, itself an object containing any number of other fields
-- All other top-level fields are ignored
 
 For example,
 
@@ -394,6 +412,8 @@ becomes
     else
       ghc-options: -O0
 
+**Note:** Conditionals with `condition: false` are omitted from the generated
+`.cabal` file.
 
 ### <a name="file-globbing"></a>File globbing
 

src/Hpack/Config.hs

@@ -56,6 +56,7 @@ module Hpack.Config (
 , Library(..)
 , Executable(..)
 , Conditional(..)
+, Cond(..)
 , Flag(..)
 , SourceRepository(..)
 , BuildType(..)
@@ -70,7 +71,6 @@ module Hpack.Config (
 , renameDependencies
 , Empty(..)
 , pathsModuleFromPackageName
-, Cond(..)
 
 , LibrarySection(..)
 , fromLibrarySectionInConditional
@@ -468,17 +468,16 @@ hasKey key (Object o) = HashMap.member key o
 hasKey _ _ = False
 
 newtype Condition = Condition {
-  _conditionCondition :: Cond
+  conditionCondition :: Cond
 } deriving (Eq, Show, Generic, FromValue)
 
-newtype Cond = Cond String
+data Cond = CondBool Bool | CondExpression String
   deriving (Eq, Show)
 
 instance FromValue Cond where
   fromValue v = case v of
-    String s -> return (Cond $ T.unpack s)
-    Bool True -> return (Cond "true")
-    Bool False -> return (Cond "false")
+    String c -> return (CondExpression $ T.unpack c)
+    Bool c -> return (CondBool c)
     _ -> typeMismatch "Boolean or String" v
 
 data ThenElse cSources cxxSources jsSources a = ThenElse {
@@ -955,7 +954,7 @@ data Section a = Section {
 } deriving (Eq, Show, Functor, Foldable, Traversable)
 
 data Conditional a = Conditional {
-  conditionalCondition :: String
+  conditionalCondition :: Cond
 , conditionalThen :: a
 , conditionalElse :: Maybe a
 } deriving (Eq, Show, Functor, Foldable, Traversable)
@@ -1277,6 +1276,13 @@ getMentionedLibraryModules (LibrarySection _ _ exposedModules generatedExposedMo
 listModules :: FilePath -> Section a -> IO [Module]
 listModules dir Section{..} = concat <$> mapM (getModules dir) sectionSourceDirs
 
+removeConditionalsThatAreAlwaysFalse :: Section a -> Section a
+removeConditionalsThatAreAlwaysFalse sect = sect {
+    sectionConditionals = filter p $ sectionConditionals sect
+  }
+  where
+    p = (/= CondBool False) . conditionalCondition
+
 inferModules ::
      FilePath
   -> String
@@ -1286,7 +1292,7 @@ inferModules ::
   -> ([Module] -> a -> b)
   -> Section a
   -> IO (Section b)
-inferModules dir packageName_ getMentionedModules getInferredModules fromData fromConditionals = traverseSectionAndConditionals
+inferModules dir packageName_ getMentionedModules getInferredModules fromData fromConditionals = fmap removeConditionalsThatAreAlwaysFalse . traverseSectionAndConditionals
   (fromConfigSection fromData [pathsModuleFromPackageName packageName_])
   (fromConfigSection (\ [] -> fromConditionals) [])
   []
@@ -1427,7 +1433,7 @@ toSection packageName_ executableNames = go
       ThenElseConditional (Product (ThenElse then_ else_) c) -> conditional c <$> (go then_) <*> (Just <$> go else_)
       FlatConditional (Product sect c) -> conditional c <$> (go sect) <*> pure Nothing
       where
-        conditional (Condition (Cond c)) = Conditional c
+        conditional = Conditional . conditionCondition
 
 type SystemBuildTool = (String, VersionConstraint)
 

src/Hpack/Render.hs

@@ -286,7 +286,13 @@ renderConditional renderSectionData (Conditional condition sect mElse) = case mE
   Nothing -> if_
   Just else_ -> Group if_ (Stanza "else" $ renderSection renderSectionData [] [] else_)
   where
-    if_ = Stanza ("if " ++ condition) (renderSection renderSectionData [] [] sect)
+    if_ = Stanza ("if " ++ renderCond condition) (renderSection renderSectionData [] [] sect)
+
+renderCond :: Cond -> String
+renderCond = \ case
+  CondExpression c -> c
+  CondBool True -> "true"
+  CondBool False -> "false"
 
 defaultLanguage :: Element
 defaultLanguage = Field "default-language" "Haskell2010"

test/EndToEndSpec.hs

@@ -48,6 +48,25 @@ spec = around_ (inTempDirectoryNamed "foo") $ do
           "package.yaml: Duplicate field $.name"
         ]
 
+    describe "handling of Paths_ module" $ do
+      it "adds Paths_ to other-modules" $ do
+        [i|
+        library: {}
+        |] `shouldRenderTo` library [i|
+        other-modules:
+            Paths_foo
+        |]
+
+      context "when Paths_ is mentioned in a conditional that is always false" $ do
+        it "does not add Paths_" $ do
+          [i|
+          library:
+            when:
+            - condition: false
+              other-modules: Paths_foo
+          |] `shouldRenderTo` library [i|
+          |]
+
     describe "spec-version" $ do
       it "accepts spec-version" $ do
         [i|

test/Helper.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ConstraintKinds #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 module Helper (
   module Test.Hspec
 , module Test.Mockery.Directory
@@ -13,6 +14,7 @@ module Helper (
 
 import           Test.Hspec
 import           Test.Mockery.Directory
+import           Data.String
 import           Control.Monad
 import           Control.Applicative
 import           System.Directory (getCurrentDirectory, setCurrentDirectory, canonicalizePath)
@@ -23,6 +25,11 @@ import           System.FilePath
 import           Data.Yaml.TH (yamlQQ)
 import           Language.Haskell.TH.Quote (QuasiQuoter)
 
+import           Hpack.Config
+
+instance IsString Cond where
+  fromString = CondExpression
+
 withCurrentDirectory :: FilePath -> IO a -> IO a
 withCurrentDirectory dir action = do
   bracket getCurrentDirectory setCurrentDirectory $ \ _ -> do

test/Hpack/ConfigSpec.hs

@@ -643,17 +643,17 @@ spec = do
       it "accepts Strings" $ do
         [yaml|
         os(windows)
-        |] `shouldDecodeTo_` Cond "os(windows)"
+        |] `shouldDecodeTo_` CondExpression "os(windows)"
 
       it "accepts True" $ do
         [yaml|
         yes
-        |] `shouldDecodeTo_` Cond "true"
+        |] `shouldDecodeTo_` CondBool True
 
       it "accepts False" $ do
         [yaml|
         no
-        |] `shouldDecodeTo_` Cond "false"
+        |] `shouldDecodeTo_` CondBool False
 
       it "rejects other values" $ do
         [yaml|