From 97f3213bb6b5e2839efac43883b25b41fe06ed6c Mon Sep 17 00:00:00 2001
From: Harrison Houghton <haro@constant.gripe>
Date: Thu, 20 Dec 2018 15:21:54 -0500
Subject: [PATCH 1/3] Generate valid Main.lhs files with `cabal init`.

Fixes haskell/cabal#5577.
---
 cabal-install/Distribution/Client/Init.hs | 19 ++++++++++++++-----
 cabal-install/changelog                   |  1 +
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/cabal-install/Distribution/Client/Init.hs b/cabal-install/Distribution/Client/Init.hs
index 8d43e32b50d..2677e7076f9 100644
--- a/cabal-install/Distribution/Client/Init.hs
+++ b/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 . fmap 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.
diff --git a/cabal-install/changelog b/cabal-install/changelog
index f7041182d2b..1400351b58e 100644
--- a/cabal-install/changelog
+++ b/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)

From b8e87b569b832dcb1ba2809564bbf141b5563a02 Mon Sep 17 00:00:00 2001
From: Mikhail Glushenkov <mikhail.glushenkov@gmail.com>
Date: Tue, 15 Jan 2019 22:30:09 +0000
Subject: [PATCH 2/3] Whitespace.

---
 cabal-install/Distribution/Client/Init.hs | 2 +-
 cabal-install/changelog                   | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/cabal-install/Distribution/Client/Init.hs b/cabal-install/Distribution/Client/Init.hs
index 2677e7076f9..8eff35d42ab 100644
--- a/cabal-install/Distribution/Client/Init.hs
+++ b/cabal-install/Distribution/Client/Init.hs
@@ -856,7 +856,7 @@ mainHs flags = (unlines . fmap prependPrefix)
     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.
diff --git a/cabal-install/changelog b/cabal-install/changelog
index 1400351b58e..df1d7db43e0 100644
--- a/cabal-install/changelog
+++ b/cabal-install/changelog
@@ -27,9 +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)

From 5540cc2c7f5a37402eb1672ee33ef651a2eff0e7 Mon Sep 17 00:00:00 2001
From: Mikhail Glushenkov <mikhail.glushenkov@gmail.com>
Date: Tue, 15 Jan 2019 22:32:33 +0000
Subject: [PATCH 3/3] s/fmap/map/

---
 cabal-install/Distribution/Client/Init.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cabal-install/Distribution/Client/Init.hs b/cabal-install/Distribution/Client/Init.hs
index 8eff35d42ab..16c82adc1e9 100644
--- a/cabal-install/Distribution/Client/Init.hs
+++ b/cabal-install/Distribution/Client/Init.hs
@@ -842,7 +842,7 @@ hasMainHs flags = case mainIs flags of
 
 -- | Default Main.(l)hs file.  Used when no Main.(l)hs exists.
 mainHs :: InitFlags -> String
-mainHs flags = (unlines . fmap prependPrefix)
+mainHs flags = (unlines . map prependPrefix)
   [ "module Main where"
   , ""
   , "main :: IO ()"