Merge pull request #5810 from hrhino/t5577

Generate valid Main.lhs files with `cabal init`.

cabal-install/Distribution/Client/Init.hs

@@ -32,7 +32,7 @@ import System.Directory
   ( getCurrentDirectory, doesDirectoryExist, doesFileExist, copyFile
   , getDirectoryContents, createDirectoryIfMissing )
 import System.FilePath
-  ( (</>), (<.>), takeBaseName, equalFilePath )
+  ( (</>), (<.>), takeBaseName, takeExtension, equalFilePath )
 import Data.Time
   ( getCurrentTime, utcToLocalTime, toGregorian, localDay, getCurrentTimeZone )
 
@@ -831,7 +831,7 @@ writeMainHs flags mainPath = do
   exists <- doesFileExist mainFullPath
   unless exists $ do
       message flags $ "Generating " ++ mainPath ++ "..."
-      writeFileSafe flags mainFullPath mainHs
+      writeFileSafe flags mainFullPath (mainHs flags)
 
 -- | Check that a main file exists.
 hasMainHs :: InitFlags -> Bool
@@ -840,14 +840,23 @@ hasMainHs flags = case mainIs flags of
              || packageType flags == Flag LibraryAndExecutable)
   _ -> False
 
--- | Default Main.hs file.  Used when no Main.hs exists.
-mainHs :: String
-mainHs = unlines
+-- | Default Main.(l)hs file.  Used when no Main.(l)hs exists.
+mainHs :: InitFlags -> String
+mainHs flags = (unlines . map prependPrefix)
   [ "module Main where"
   , ""
   , "main :: IO ()"
   , "main = putStrLn \"Hello, Haskell!\""
   ]
+  where
+    prependPrefix "" = ""
+    prependPrefix line
+      | isLiterate = "> " ++ line
+      | otherwise  = line
+    isLiterate = case mainIs flags of
+      Flag mainPath -> takeExtension mainPath == ".lhs"
+      _             -> False
+
 
 -- | Move an existing file, if there is one, and the overwrite flag is
 --   not set.

cabal-install/changelog

@@ -14,6 +14,7 @@
 	  error message, but with an increase in run time. (#5647)
 	* v2-test now succeeds when there are no test suites. (#5435)
 	* Add '--lib', '--exe', and '--libandexe' shorthands to init. (#5759)
+	* init now generates valid `Main.lhs` files. (#5577)
 
 2.4.1.0 Mikhail Glushenkov <mikhail.glushenkov@gmail.com> November 2018
 	* Add message to alert user to potential package casing errors. (#5635)
@@ -26,9 +27,9 @@
 	* Register monolithic packages installed into the store due to a
 	  build-tool dependency if they also happen to contain a buildable
 	  public lib. (#5379,#5604)
-        * Fixed a Windows bug where cabal-install tried to copy files
-          after moving them (#5631).
-        * 'cabal v2-repl' now works for indefinite (in the Backpack sense) components. (#5619)
+	* Fixed a Windows bug where cabal-install tried to copy files
+	  after moving them (#5631).
+	* 'cabal v2-repl' now works for indefinite (in the Backpack sense) components. (#5619)
 	* Set data dir environment variable for tarballs and remote repos (#5469)
 	* Fix monolithic inplace build tool PATH (#5633)
 	* 'cabal init' now supports '-w'/'--with-compiler' flag (#4936, #5654)