resolve T13704

Summary: allow -main-is to change export list for default module
header, allowing one to change the entry point to one's program.

Test Plan: ./validate

Reviewers: bgamari, nomeata, mpickering

Reviewed By: mpickering

Subscribers: mpickering, rwbarton, carter

GHC Trac Issues: #13704

Differential Revision: https://phabricator.haskell.org/D5189

5 files changed, 45 insertions, 9 deletions

diff --git a/.gitignore b/.gitignore
index c72d044473..83fa7ececb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -208,4 +208,10 @@ GIT_COMMIT_ID
 # -----------------------------------------------------------------------------
 # Output of ghc-in-ghci
 
-/.ghci-objects/
\ No newline at end of file
+/.ghci-objects/
+
+# -----------------------------------------------------------------------------
+# ghc.nix
+ghc.nix/
+
+
diff --git a/compiler/typecheck/TcRnExports.hs b/compiler/typecheck/TcRnExports.hs
index dbe2b4b22b..1b57608a41 100644
--- a/compiler/typecheck/TcRnExports.hs
+++ b/compiler/typecheck/TcRnExports.hs
@@ -33,7 +33,7 @@ import DataCon
 import PatSyn
 import Maybes
 import Util (capitalise)
-
+import FastString (fsLit)
 
 import Control.Monad
 import DynFlags
@@ -124,19 +124,18 @@ tcRnExports explicit_mod exports
        -- list, to avoid bleating about re-exporting a deprecated
        -- thing (especially via 'module Foo' export item)
    do   {
-        -- If the module header is omitted altogether, then behave
-        -- as if the user had written "module Main(main) where..."
-        -- EXCEPT in interactive mode, when we behave as if he had
+        -- In interactive mode, we behave as if he had
         -- written "module Main where ..."
-        -- Reason: don't want to complain about 'main' not in scope
-        --         in interactive mode
         ; dflags <- getDynFlags
+        ; let default_main = case mainFunIs dflags of
+                 Just main_fun -> mkUnqual varName (fsLit main_fun)
+                 Nothing       -> main_RDR_Unqual
         ; let real_exports
                  | explicit_mod = exports
                  | ghcLink dflags == LinkInMemory = Nothing
                  | otherwise
                           = Just (noLoc [noLoc (IEVar noExt
-                                     (noLoc (IEName $ noLoc main_RDR_Unqual)))])
+                                     (noLoc (IEName $ noLoc default_main)))])
                         -- ToDo: the 'noLoc' here is unhelpful if 'main'
                         --       turns out to be out of scope
 
diff --git a/docs/users_guide/bugs.rst b/docs/users_guide/bugs.rst
index aee8dc5842..0290622b62 100644
--- a/docs/users_guide/bugs.rst
+++ b/docs/users_guide/bugs.rst
@@ -173,6 +173,34 @@ same context. For example, this is fine: ::
 
 .. _infelicities-Modules:
 
+Default Module headers with -main-is
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Haskell2010 report specifies in <https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-990005.1> that
+
+    "An abbreviated form of module, consisting only of the module body,
+     is permitted. If this is used, the header is assumed to be
+     `module Main(main) where`."
+
+Consider the following program: ::
+
+    -- file: Main.hs
+    program :: IO ()
+    program = return ()
+
+Under the report, this would fail with ``ghc -main-is Main.program Main.hs``
+with the following errors: ::
+
+    Main.hs:1:1: error:
+        Not in scope: 'main'
+        Perhaps you meant 'min' (imported from Prelude)
+
+    Main.hs:1:1: error:
+        The main IO action 'program' is not exported by module 'Main'
+
+GHC's flag '-main-is' allows one to change the entry point name so that
+the above example would succeed.
+
 Module system and interface files
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/testsuite/tests/module/T13704.hs b/testsuite/tests/module/T13704.hs
new file mode 100644
index 0000000000..38b1cb73fc
--- /dev/null
+++ b/testsuite/tests/module/T13704.hs
@@ -0,0 +1,3 @@
+program = return ()
+
+-- meant to be compiled with 'ghc -main-is Main.program T13704.hs'
diff --git a/testsuite/tests/module/all.T b/testsuite/tests/module/all.T
index e862413897..dbba44f0ba 100644
--- a/testsuite/tests/module/all.T
+++ b/testsuite/tests/module/all.T
@@ -284,4 +284,4 @@ test('T11970B', normal, compile_fail, [''])
 test('MultiExport', normal, compile, [''])
 test('T13528', normal, compile, [''])
 test('T13622', normal, compile, [''])
-
+test('T13704', normal, compile, ['-main-is Main.program'])