Add a README to the files generated by cabal init

See #11231

Author: Kleidukos

cabal-install/src/Distribution/Client/Init/Defaults.hs

@@ -23,6 +23,7 @@ module Distribution.Client.Init.Defaults
   , defaultLicenseIds
   , defaultMainIs
   , defaultChangelog
+  , defaultReadme
   , defaultCategories
   , defaultInitFlags
   , defaultLanguage
@@ -75,6 +76,9 @@ defaultPackageType = Executable
 defaultChangelog :: FilePath
 defaultChangelog = "CHANGELOG.md"
 
+defaultReadme :: FilePath
+defaultReadme = "README.md"
+
 defaultLicense :: CabalSpecVersion -> SpecLicense
 defaultLicense csv
   | csv < CabalSpecV2_2 = SpecLicense $ Right AllRightsReserved

cabal-install/src/Distribution/Client/Init/FileCreators.hs

@@ -18,6 +18,7 @@ module Distribution.Client.Init.FileCreators
   ( -- * Commands
     writeProject
   , writeLicense
+  , writeReadme
   , writeChangeLog
   , prepareLibTarget
   , prepareExeTarget
@@ -70,6 +71,7 @@ writeProject (ProjectSettings opts pkgDesc libTarget exeTarget testTarget)
 
       writeLicense opts pkgDesc
       writeChangeLog opts pkgDesc
+      writeReadme opts pkgDesc
 
       let pkgFields = mkPkgDescription opts pkgDesc
           commonStanza = mkCommonStanza opts
@@ -240,6 +242,28 @@ writeChangeLog opts pkgDesc
     go =
       void $ writeFileSafe opts defaultChangelog changeLog
 
+writeReadme :: Interactive m => WriteOpts -> PkgDescription -> m ()
+writeReadme opts pkgDesc
+  | Just docs <- _pkgExtraDocFiles pkgDesc
+  , defaultReadme `Set.member` docs =
+      go
+  | defaultReadme `elem` _pkgExtraSrcFiles pkgDesc = go
+  | otherwise = return ()
+  where
+    readme =
+      unlines
+        [ "# " ++ prettyShow (_pkgName pkgDesc)
+        , ""
+        , "## Build"
+        , ""
+        , "Run `$ cabal build` to build the project"
+        , "## Documentation"
+        , ""
+        , "Run `$ cabal haddock --open` to generate a reference for the API of the project."
+        ]
+    go =
+      void $ writeFileSafe opts defaultReadme readme
+
 -- -------------------------------------------------------------------- --
 -- Utilities
 

cabal-install/tests/UnitTests/Distribution/Client/Init/Simple.hs

@@ -168,7 +168,7 @@ simplePkgDesc pkgName =
     ""
     ""
     mempty
-    (Just $ Set.singleton defaultChangelog)
+    (Just $ Set.fromList [defaultReadme, defaultChangelog])
 
 simpleLibTarget :: [Dependency] -> LibTarget
 simpleLibTarget baseDep =