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, 4 insertions, 1 deletions

diff --git a/compiler/main/DriverMkDepend.hs b/compiler/main/DriverMkDepend.hs
index 03545d4828..1b4d1ac3f1 100644
--- a/compiler/main/DriverMkDepend.hs
+++ b/compiler/main/DriverMkDepend.hs
@@ -248,7 +248,7 @@ findDependency hsc_env srcloc pkg imp is_boot include_pkg_deps
                 -- we've done it once during downsweep
           r <- findImportedModule hsc_env imp pkg
         ; case r of
-            Found loc _
+            FoundModule (FoundHs { fr_loc = loc })
                 -- Home package: just depend on the .hi or hi-boot file
                 | isJust (ml_hs_file loc) || include_pkg_deps
                 -> return (Just (addBootSuffix_maybe is_boot (ml_hi_file loc)))
@@ -257,6 +257,9 @@ findDependency hsc_env srcloc pkg imp is_boot include_pkg_deps
                 | otherwise
                 -> return Nothing
 
+            -- TODO: FoundSignature.  For now, we assume home package
+            -- "signature" dependencies look like FoundModule.
+
             fail ->
                 let dflags = hsc_dflags hsc_env
                 in throwOneError $ mkPlainErrMsg dflags srcloc $