Support for multiple signature files in scope.

Summary:
A common pattern when programming with signatures is to combine multiple
signatures together (signature linking).  We achieve this by making it
not-an-error to have multiple, distinct interface files for the same module
name, as long as they have the same backing implementation.  When a user
imports a module name, they get ALL matching signatures dumped into their
scope.

On the way, I refactored the module finder code, which now distinguishes
between exact finds (when you had a 'Module') and regular finds (when
you had a 'ModuleName').  I also refactored the package finder code to
use a Monoid instance on LookupResult to collect together various results.

ToDo: At the moment, if a signature is declared in the local package,
it completely overrides any remote signatures.  Eventually, we'll want
to also pull in the remote signatures (or even override the local signature,
if the full implementation is available.)  There are bunch of ToDos in the
code for what to do once this is done.

ToDo: At the moment, whenever a module name lookup occurs in GHCi and we
would have seen a signature, we instead continue and return the Module
for the backing implementation.  This is correct for most cases, but there
might be some situations where we want something a little more fine-grained
(e.g. :browse should only list identifiers which are available through
the in-scope signatures, and not ALL of them.)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, hvr, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D790

GHC Trac Issues: #9252

1 files changed, 30 insertions, 16 deletions

diff --git a/compiler/ghci/Linker.hs b/compiler/ghci/Linker.hs
index 9446e3df2b..cec09048c2 100644
--- a/compiler/ghci/Linker.hs
+++ b/compiler/ghci/Linker.hs
@@ -562,23 +562,29 @@ getLinkDeps hsc_env hpt pls replace_osuf span mods
 
         -- 3.  For each dependent module, find its linkable
         --     This will either be in the HPT or (in the case of one-shot
-        --     compilation) we may need to use maybe_getFileLinkable
+        --     compilation) we may need to use maybe_getFileLinkable.
+        --     If the module is actually a signature, there won't be a
+        --     linkable (thus catMaybes)
       ; let { osuf = objectSuf dflags }
-      ; lnks_needed <- mapM (get_linkable osuf) mods_needed
+      ; lnks_needed <- fmap Maybes.catMaybes
+                     $ mapM (get_linkable osuf) mods_needed
 
       ; return (lnks_needed, pkgs_needed) }
   where
     dflags = hsc_dflags hsc_env
     this_pkg = thisPackage dflags
 
-        -- The ModIface contains the transitive closure of the module dependencies
-        -- within the current package, *except* for boot modules: if we encounter
-        -- a boot module, we have to find its real interface and discover the
-        -- dependencies of that.  Hence we need to traverse the dependency
-        -- tree recursively.  See bug #936, testcase ghci/prog007.
-    follow_deps :: [Module]             -- modules to follow
-                -> UniqSet ModuleName         -- accum. module dependencies
-                -> UniqSet PackageKey          -- accum. package dependencies
+    -- | Given a list of modules @mods@, recursively discover all external
+    -- package and local module (according to @this_pkg@) dependencies.
+    --
+    -- The 'ModIface' contains the transitive closure of the module dependencies
+    -- within the current package, *except* for boot modules: if we encounter
+    -- a boot module, we have to find its real interface and discover the
+    -- dependencies of that.  Hence we need to traverse the dependency
+    -- tree recursively.  See bug #936, testcase ghci/prog007.
+    follow_deps :: [Module]                     -- modules to follow
+                -> UniqSet ModuleName           -- accum. module dependencies
+                -> UniqSet PackageKey           -- accum. package dependencies
                 -> IO ([ModuleName], [PackageKey]) -- result
     follow_deps []     acc_mods acc_pkgs
         = return (uniqSetToList acc_mods, uniqSetToList acc_pkgs)
@@ -601,6 +607,7 @@ getLinkDeps hsc_env hpt pls replace_osuf span mods
                     where is_boot (m,True)  = Left m
                           is_boot (m,False) = Right m
 
+            -- Boot module dependencies which must be processed recursively
             boot_deps' = filter (not . (`elementOfUniqSet` acc_mods)) boot_deps
             acc_mods'  = addListToUniqSet acc_mods (moduleName mod : mod_deps)
             acc_pkgs'  = addListToUniqSet acc_pkgs $ map fst pkg_deps
@@ -631,30 +638,37 @@ getLinkDeps hsc_env hpt pls replace_osuf span mods
 
     get_linkable osuf mod_name      -- A home-package module
         | Just mod_info <- lookupUFM hpt mod_name
-        = adjust_linkable (Maybes.expectJust "getLinkDeps" (hm_linkable mod_info))
+        = adjust_linkable (hm_iface mod_info)
+            (Maybes.expectJust "getLinkDeps" (hm_linkable mod_info))
         | otherwise
         = do    -- It's not in the HPT because we are in one shot mode,
                 -- so use the Finder to get a ModLocation...
+                -- ezyang: I don't actually know how to trigger this codepath,
+                -- seeing as this is GHCi logic. Template Haskell, maybe?
              mb_stuff <- findHomeModule hsc_env mod_name
              case mb_stuff of
-                  Found loc mod -> found loc mod
+                  FoundExact loc mod -> found loc mod
                   _ -> no_obj mod_name
         where
             found loc mod = do {
                 -- ...and then find the linkable for it
                mb_lnk <- findObjectLinkableMaybe mod loc ;
+               iface <- initIfaceCheck hsc_env $
+                            loadUserInterface False (text "getLinkDeps2") mod ;
                case mb_lnk of {
                   Nothing  -> no_obj mod ;
-                  Just lnk -> adjust_linkable lnk
+                  Just lnk -> adjust_linkable iface lnk
               }}
 
-            adjust_linkable lnk
+            adjust_linkable iface lnk
+                -- Signatures have no linkables! Don't return one.
+                | Just _ <- mi_sig_of iface = return Nothing
                 | Just new_osuf <- replace_osuf = do
                         new_uls <- mapM (adjust_ul new_osuf)
                                         (linkableUnlinked lnk)
-                        return lnk{ linkableUnlinked=new_uls }
+                        return (Just lnk{ linkableUnlinked=new_uls })
                 | otherwise =
-                        return lnk
+                        return (Just lnk)
 
             adjust_ul new_osuf (DotO file) = do
                 MASSERT(osuf `isSuffixOf` file)