improve the cyclic module error message as per #1856

 - remove the main: stuff
 - show the filenames
 - don't clutter the output with imports that aren't involved in
   the cycle

diff --git a/compiler/main/GHC.hs b/compiler/main/GHC.hs
index 3728838..0be0fc4 100644 (file)
--- a/compiler/main/GHC.hs
+++ b/compiler/main/GHC.hs
@@ -2318,12 +2318,19 @@ cyclicModuleErr ms
   = hang (ptext (sLit "Module imports form a cycle for modules:"))
        2 (vcat (map show_one ms))
   where
-    show_one ms = sep [ show_mod (ms_hsc_src ms) (ms_mod ms),
-                       nest 2 $ ptext (sLit "imports:") <+> 
-                                  (pp_imps HsBootFile (ms_srcimps ms)
-                                  $$ pp_imps HsSrcFile  (ms_imps ms))]
+    mods_in_cycle = map ms_mod_name ms
+    imp_modname = unLoc . ideclName . unLoc
+    just_in_cycle = filter ((`elem` mods_in_cycle) . imp_modname)
+
+    show_one ms = 
+           vcat [ show_mod (ms_hsc_src ms) (ms_mod_name ms) <+>
+                  maybe empty (parens . text) (ml_hs_file (ms_location ms)),
+                  nest 2 $ ptext (sLit "imports:") <+> vcat [
+                     pp_imps HsBootFile (just_in_cycle $ ms_srcimps ms),
+                     pp_imps HsSrcFile  (just_in_cycle $ ms_imps ms) ]
+                ]
     show_mod hsc_src mod = ppr mod <> text (hscSourceString hsc_src)
-    pp_imps src mods = fsep (map (show_mod src) mods)
+    pp_imps src imps = fsep (map (show_mod src . unLoc . ideclName . unLoc) imps)
 
 
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