Change loadSrcInterface to return a list of ModIface

Summary:
This change is in preparation to support signature imports, which may pull in
multiple interface files.  At the moment, the list always contains only one
element, but in a later patch it may contain more.

I also adjusted some error reporting code so that it didn't take the full
iface, but just whether or not the iface in question was a boot module.

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

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D436

1 files changed, 44 insertions, 6 deletions

diff --git a/compiler/iface/LoadIface.lhs b/compiler/iface/LoadIface.lhs
index 3b2f7f25c9..250ef2f182 100644
--- a/compiler/iface/LoadIface.lhs
+++ b/compiler/iface/LoadIface.lhs
@@ -78,26 +78,61 @@ import System.FilePath
 %************************************************************************
 
 \begin{code}
+-- Note [Un-ambiguous multiple interfaces]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- When a user writes an import statement, this usually causes a *single*
+-- interface file to be loaded.  However, the game is different when
+-- signatures are being imported.  Suppose in packages p and q we have
+-- signatures:
+--
+--  module A where
+--      foo :: Int
+--
+--  module A where
+--      bar :: Int
+--
+-- If both packages are exposed and I am importing A, I should see a
+-- "unified" signature:
+--
+--  module A where
+--      foo :: Int
+--      bar :: Int
+--
+-- The way we achieve this is having the module lookup for A load and return
+-- multiple interface files, which we will then process as if there were
+-- "multiple" imports:
+--
+--  import "p" A
+--  import "q" A
+--
+-- Doing so does not cause any ambiguity, because any overlapping identifiers
+-- are guaranteed to have the same name if the backing implementations of the
+-- two signatures are the same (a condition which is checked by 'Packages'.)
+
+
 -- | Load the interface corresponding to an @import@ directive in 
 -- source code.  On a failure, fail in the monad with an error message.
+-- See Note [Un-ambiguous multiple interfaces] for why the return type
+-- is @[ModIface]@
 loadSrcInterface :: SDoc
                  -> ModuleName
                  -> IsBootInterface     -- {-# SOURCE #-} ?
                  -> Maybe FastString    -- "package", if any
-                 -> RnM ModIface
+                 -> RnM [ModIface]
 
 loadSrcInterface doc mod want_boot maybe_pkg
   = do { res <- loadSrcInterface_maybe doc mod want_boot maybe_pkg
        ; case res of
-           Failed err      -> failWithTc err
-           Succeeded iface -> return iface }
+           Failed err       -> failWithTc err
+           Succeeded ifaces -> return ifaces }
 
--- | Like 'loadSrcInterface', but returns a 'MaybeErr'.
+-- | Like 'loadSrcInterface', but returns a 'MaybeErr'.  See also
+-- Note [Un-ambiguous multiple interfaces]
 loadSrcInterface_maybe :: SDoc
                        -> ModuleName
                        -> IsBootInterface     -- {-# SOURCE #-} ?
                        -> Maybe FastString    -- "package", if any
-                       -> RnM (MaybeErr MsgDoc ModIface)
+                       -> RnM (MaybeErr MsgDoc [ModIface])
 
 loadSrcInterface_maybe doc mod want_boot maybe_pkg
   -- We must first find which Module this import refers to.  This involves
@@ -106,9 +141,12 @@ loadSrcInterface_maybe doc mod want_boot maybe_pkg
   -- interface; it will call the Finder again, but the ModLocation will be
   -- cached from the first search.
   = do { hsc_env <- getTopEnv
+       -- ToDo: findImportedModule should return a list of interfaces
        ; res <- liftIO $ findImportedModule hsc_env mod maybe_pkg
        ; case res of
-           Found _ mod -> initIfaceTcRn $ loadInterface doc mod (ImportByUser want_boot)
+           Found _ mod -> fmap (fmap (:[]))
+                        . initIfaceTcRn
+                        $ loadInterface doc mod (ImportByUser want_boot)
            err         -> return (Failed (cannotFindInterface (hsc_dflags hsc_env) mod err)) }
 
 -- | Load interface directly for a fully qualified 'Module'.  (This is a fairly